Training for extra positions is a very poor strategy.
If you want multi positional players, get them off the market. There is usually plenty of older players to choose from that can play 2 or 3 positions.

I'm at level 12, and have one guy from level 5 and another from level 4, i never trained them for an ability (they both came with one), or a position. Since they were long term starters and their position was fixed, it would have never occurred to me to do such a thing.