You can unlock two special bonus teams, the Dingers and the Islanders, by going into the Exhibition Mode and tapping R2+L2 at the team selection screen. Once you hear a gong sound, you will then be able to select either team.

If you don't hear the tone at first, tap the buttons until you do.

To get the bonus team, the Austin Dingers, in Batting Practice enter that mode and then press R2+R1+L2+L1 at the team selection screen.

Press L1 or R1 in Exhibition mode.