Pretty much no one is doing this. It's just more complicated than you want it to be.
You're not simply acquiring future-HOF defenseman Erik Karlsson.
First, the trading with an extension in hand thing is unlikely to work out, unless it's the team that Karlsson has his heart set on, in which case, he and the team in question now have leverage over the Sens to lower the price, as Karlsson can negotiate an extension with that one team only, and now the trade market has been cut down to exactly one team. Remember, it benefits Karlsson that whichever team that acquires him doesn't pay too much. Dorion is now in an even more impossible situation.
So, more than likely, you are trading for Karlsson without the extension, meaning you are only getting him for 1 year, and then you're trading for the opportunity to compete with every other team in the league to sign him to a massive cap-busting contract (which you'll do, because it's Erik ****ing Karlsson, but still) and oh yeah, there are concerns that his ankle injury is going to permanently affect his speed as he enters his 30s(!), robbing him of one of his best weapons. And you're going to pay a premium for this privilege. That's a dicey proposition for every team in the league. For the Isles, who just went through hell with Tavares, that's nightmarish.