In order to play Owner's mode without the cap, first you must enter Owner's mode and create a player. Sign him to your team, then bring your team over the salary cap (the cap room is in the negatives).
Go back and edit your player's attributes (one point difference in anything will do). The game will notify you that this will bring the team over the cap, that the cap will be turned off, and will ask you if you want to proceed. Select yes.
The salary cap will be turned off, but Owner's mode will not. This takes off the burden of cap room economics. Have fun!