The NHL has to appear before the National Labor Relations Board and whatever Canadian office equivalent and declare an impasse first. Once the NLRB and Canadian offices grant the league this impasse, then the NHL can institute the salary cap they are looking for. If the NHLPA decide to strike against the owners as a reaction to this cap, then the NHL can hire replacement players.
I am assuming that the NFL went through this process in the past. I could be wrong. If it was a lockout, then they only had to face the NLRB, since there are no Canadian NFL teams. If it was only a player strike, then they could immediately go to replacement players if they wanted. The NHL would have to face any Canadian powers that be, aside from the NLRB, to get their impasse. I don't know if Canada has a national office for labor relations or if there are provincial offices.