Yes, they can, but I believe it's highly unlikely, almost impossible, that this would happen. I don't believe they would make it up to the spawning pools to mate.
I agree with Nell. It's a requirement. Introductions and transfers are administered through DFO. We are tightly controlled on the strain of fish that we're allowed to farm in our cages. It is very much the same fish. One of my first experiences with salmon farming was with a geneticist from the Atlantic Salmon Federation while I was at the University of Guelph. He did some population genetics. We will try to have genetic improvement in our stocks, but it's going to be a very slow process. Even though we are maybe seven generations removed from wild fish, I don't believe there has been a significant change in genetics.