hmmm...Not sure if I quite understand why you want to do this. Is this so you can play your already trained players in their desired spots as well as play trainees in that spot?
lets make an example, you normally train "just Center", this works fine with 3 trainees and as PF you play with a high skilled Center/PF. Now you notice, my young center are fine but they also need rebounding ->you have train PF/C which brings you in trouble because your "PF" shouldn't train(and don't play well at SF) and you just have those three trainees. What should ya do?
Another scenario, is that you just want to train in two of your three games, and make a damn strong lineup in the third game - somehthing which happens often at least in germany(we are kings in giving up our away games).