No, the Reaper man is definately right, I created several lower league teams on my game and to bolster their squads I added a few average unused International players, say from Belgium or Japan to them in the same way he stated.
Selecting register player then the team I want to add to - Selecting Add - Selecting the national side I wanted a player from - then selecting particular player.
The only time it isn't possible is if the player plays for a club in the game, in this case I have to go to that team instead or if the team i'm adding to are full.
What format are you on?