Turn the salary cap off, find a player you want (for example, Calvin Johnson).

Select him, go to "Edit Player", and change him to a QB. His rating will drop to 12.

Press Start, and choose to keep the player without saving. Trade your lowest rated WR for him. The trade will be accepted.

Then, go back in and change him back to a WR. You will now have Calvin Johnson without losing an important player.

Repeat this process to create an all-star team.