Yes, you can do this.

1st. Hit END to open Coin Door

2nd. using 7,8,9,0 (Escape, Down, Up and Enter respectively) to move around in the menus. Hit 0 twice

3rd. Using 8 and 9 find a menu called Adjustments

4th. Enter Feature Adjustments and Find which menu holds the number of balls.

5th. Using 0 to select and 8,9 to adjust make your changes then press 0 again to save.

6th. Keep presing 7 until you exit all the menus and are back to the game. Then press END again to close the door and viola!