All you have to do to get coaches is create a new player and level him up using the fast player creator editor thing. You'll unlock the coaches depending on how you level up the playstyles.
There's no way to unlock more coaches when you reach level 20. So if your player A is level 20, that's it for the coaches. What you can do is create another player (player B) and depending on how you spend the XP's you'll get coaches specifically designated for player B.
I had this same question way back when
hope this clears it up