Walk up to Vegeta and ask him what the scouter says about your power level. If he says "IT'S OVER 9000!!!", shout to the TV screen, yell "WHAT?!? 9000?!?!?" and do 30 Jumping-Jacks. You will unlock Super Saiyan EVERYONE.
Don't you hate these fake cheats? Here's a list of the only playable characters...
Mr. Game & Watch
And yes, the only reason I did this is so I could make up a fake cheat of my own. Then say it's wrong. :D