That is possible but I find it highly unlikely the NHL would be more restrictive than a league like the NFL. EA already allows you to edit players in the season mode so it really doesn't seem like some rep from the NHL is barring them from doing that in their franchise mode. Maybe for something like 2K's "era's" mode, that is harder to work with NHLPA but that's a distant pipe-dream for EA to implement.
Is the NHL really restricting the ability for them to do what literally every other sports game allows? They didn't restrict 2K from doing it back when they were making video games. I just find that very hard to believe that the NHL has the ability to tell EA to restrict the ability to edit generated prospects in an completely offline mode.
If anything, I'd wager the NHL is far more hands off than any other league when it comes to their sports game.