No they cannot.
An employer cannot simply use replacement players (or replacement workers for that matter). If you could no company would ever negotiate, they would simply restart each time and avoid negotiating with the union. the goal of labour law is collective bargaining to reach an agreement.
Also bear in mind the NHL is trans-border operation. In the US the applicable labour law is federal and under the auspices of the NLRB. In Canada the jurisdiction is provincial and subject to the labour codes of the home provinces of the Canadian NHL teams (BC, Alberta, Ontario and Quebec) and the respective provincial labour relations boards.
In the US to get to replacement workers, you must make sure there is in fact a bargaining impasse before you declare it, because if you are wrong, the financial liabilities can be horrendous. A declaration is only possible after complete deadlock and after making your final best offer to the union. The offer must be comprehensive (in other words a replacement CBA), if rejected by the union then you must implement that offer as the temporary CBA. Only then you are entitled to re-open and try to get replacements to run your business.
http://www.lawmemo.com/emp/articles/hardball.htm
BASIC PRINCIPLE - a bargaining impasse and use of replacements is a
TEMPORARY measure and the labour dispute continues. That has some very interesting immigration law implications in the US and Canada noted below.
The union has several options if the employer declares an impasse. The union can then strike and/or seek a ruling from the NLRB that the employer committed an unfair labour practise (e.g there was in fact no bargaining impasse). The NLRB upholds very few impasse declarations.
In 1994 MLB declared an impasse, imposed a CBA and opened with "Scab Ball" after lengthy face to face negotiations, significant movement by the MLB owners on a number of issues, use of a federal mediator and even an attempt by President Clinton to mediate. MLB lost at the NLRB and were ordered to play for two more years under the previous CBA.
An impasse declaration is not easy to get confirmed by the NLRB and if the owners lose they are on the hook for the salaries owing to the players during the time they operated under the voided declaration of impasse.
BTW in the 1994 labour dispute replacement players could not be used by the Toronto Blue Jays or Montreal Expos as Canadian provincial labour laws prohibited the use of replacement workers absolutely. Subsequently the law was changed in Ontario by the Harris government but it has been proposed by the current government to re-impose the ban on replacement workers.
In Canada there is no procedure for an impasse declaration. Currently both BC (Vancouver Canucks) and Quebec (Montreal Canadians) prohibit the use of replacement workers under their labour codes. In Quebec the Habs players have not formally certified themselves as labour union local so technically replacement players might be possible but a quick 5 minute vote as occurred with the Expos in 1994 would bring the replacement worker ban completely into effect. As noted the ban on replacement workers may be re-imposed in Ontario and that would cover the Leafs and Senators. That would leave Calgary and Edmonton but as noted there is no procedure for impasse declaration and there is no guarantee the Alberta Labour Board would allow replacement players.
Another option the NHLPA has is decertification. In the last disputes with the NBA and MLB, the player associations threatened decertification and the owners backed down. Decertification is often referred to a s nuclear deterrent because the players lose the protection of the union and is likely a last resort tactic but a very effective one if used properly.
The reason the owners fear decertification is that decertification would result in the loss of protection from anti-trust law in the US and competition law in Canada. Pro sports can operate in breach of anti-trust law only so long as they have a CBA to protect them and decertification ends that. In 1950 the US Supreme Court ruled that anti-trust law applied to pro sports (except baseball which had an exemption) in the case of Radovich v. NFL. If the owners breach anti-trust law which would include such things as an Entry Draft, reserve clause for players (i.e. players become free agents at the end of each contract without such a clause) or any other non-competitive practise then the NHL would be liable for "treble (triple) damages" under US laws and the US courts are very quick to rule against breaches of anti-trust law.
In Canada we have similar law under the Competition Act, An Act to provide for the general regulation of trade and commerce in respect of conspiracies, trade practices and mergers affecting competition. Fines to be imposed in Canada have no upper limit and are at the discretion of the court. Canadian law has specific provisions that apply to professional sports IF there is no CBA:
In 1987 the NFL "broke" the union with replacement players but the players ultimately prevailed by decertifying and won free agency as one of the concessions. It also generated a couple of hundred anti-trust actions which but for one the owners finally settled in return for granting free agency to the players along with other concessions.
For a detailed review of the decertification process in that dispute see this material from the one leading sports and entertainment labour law experts, Paul Weiler (a Canadian by birth whose brother, Joe teaches labour and sports law at the University of BC law school and who was Chairman of the BC labour Relations Board in the past)who is a professor at Harvard law school. For everything you ever wanted to know about sports labour law, anti-trust law, etc but were afraid to ask see Professor Weiler's 380 page book, "Levelling the Playing Field" online at:
http://www.hup.harvard.edu/educators/pdf/weilex.pdf
For the actual NFL decertification information see pages 108-110.
For a recap of the 1994 hockey lockout see pages 111-112.
For anti-trust law and how it interrelates to pro sports see pages 150-169.
So if the NHL declares an impasse they better pray they get it right because the consequences of being wrong are horrendous. This is probably why Bettman has continually denied the NHL plans to use replacement players. He is after all a labour lawyer himself and he would know the potential consequences of a botched declaration of impasse.
As noted above there is another wrinkle that faces the NHL - immigration law. Because it operates in two countries and many of its players are neither Canadian nor American, immigration laws in Canada and the US will determine who can play as a replacement player.
Bear in mind as noted previously an impasse declaration and use of replacement is considered only a temporary measure and therefore the labour dispute continues. Under Canadian immigration law for a foreign player to play in Canada (Europeans and Americans) they must have a work permit issued from Canada Immigration unless they have been admitted an immigrant previously. Under US immigration law for a foreign player to play in US (Europeans and Canadians) they must have a work permit issued from Canada Immigration unless they have been admitted an alien resident previously.
Under the immigration laws of both countries, a work permit cannot be issued to a foreign national where there is a labour dispute in progress. So only Canadian citizens and permanent residents can play for Canadian based teams and only US citizens and alien residents can play for US based teams - no European players at all.
Not quite a simple as some would have you believe, eh wot?
Ain't labour, sports and anti-trust law fun???? It certainly keeps a battery of lawyers employed by the NHL and NHLPA.