To get an bigger roster without beating all the teams, go to the title screen and press the following buttons: Start, C, Up, Down, B, Left, A, Right, Down, Start. When it asks you to enter your initials select Yes and enter the initial Jam.
Enter the following sequence at the Tonight's Match Up screen.
Power Up - Code
Dunks - Left, Right, A, B, B, A
3 pointers - Up, Down, Left, Right, Left, Down, Up
Turbo - B, B, B, A, Down, Down, Up, Left