Secret Players
While you can't unlock celebrities the way the console versions of NBA Jam allow, there are hidden characters that can be unlocked at the initials screen. All of the secret players are actually the creators of NBA Jam. For example, entering the initials "SAL" with the birthday of Feb. 1 will unlock a character named Sal DiVita. A full list of the unlockable characters and their codes can be found below in the Game FAQs link.
Tonight's Matchup
A number of cheats can be triggered at the "Tonight's Matchup" screen. For power-up defense, press the block or steal button eight times (before the screen goes dark). For power-up interceptions, hold the joystick down and press all of the buttons until the tipoff. For power-up offense, you have to be really fast--press the block or steal button 21 times before the screen goes dark. For a full list of "Tonight's Matchup" cheats, see the Game FAQs link below.
Big Heads
This is a silly cheat that doesn't make the game any easier or harder, but it's fun to play with characters that look like bobbleheads. This cheat must also be put in at the "Tonight's Matchup" screen. Hold the joystick up and press the turbo button and the steal button until tipoff. The heads of all of the players will balloon to an absurd size. This won't effect gameplay but will make for a more goofy NBA Jam game.