When you put in your match orders and formations just select what position you want the player to be at. He will then play that position. Over time the player will improve his BPOS score in that position, when it exceeds his current highest BPOS rating he will then be listed at that position on the BPOS rating.

For example, I have a player that is now playing as TE or WR(depending on the formation I am running), but his BPOS still shows him as OL. My current QB started out as an OL, but now shows as QB.

You can also easily train a player to be 5 stars (depending on talent) in a number of positions. For example I am creating a flex player who just happens to have 17 speed and strength. So I am playing him at WR, then I will play him at Running Back and then I will play him at Tight End. Thus giving him 5 stars rating in three positions.

Obviously this takes time, 42 weeks to be precise, but that's around 2 and a half seasons.