TeewhY is right, if you want your guard to train inside skills, he has to play at the position you choose from the Training menu (PF, C, whatever). So he might be really bad in your game at that position, but at least he'll get training.
Quite honestly, I wouldn't worry about cross-training players like that for a while. I would focus on getting guards the best guard skills that you can. If you have any questions (I'm the NZ National Team coach by the way), feel free to respond or send me a BB-mail!