Add/Remove leagues.

Say if I add a new league to an existing save through to in game option. Will the real life players that play in those leagues be available? I started a save with a bunch of leagues on view only. Thinking that the players I'd like to sign one day wouldn't be on the game. But I just searched for one of them and the team that he plays for has all their players "greyed" out.. will adding the league as playable bring the player in to existence?

Yes, and if you do so after a number of years, not only will the real players appear, but some of them will have moved clubs and there will also be a number of regens added to the squad.