So.. You want to train one player, but not the others that get minutes at the same position?
Well theoretically you could- it's not a good idea IMO because, lets face it, training can't HURT for anybody. If you are sure you want to do that, put the player as starter, backup, and Reserve for the position during the game, and have only 5 players in each game, putting the rest on reserve (right column on the set lineup page), and you should be able to avoid giving any other players training at that position.
I don't see why you'd want to do this though.