i dont think you can

There are no cheats, except the Pro league thing, where you hold L1+R1 on the Hard level
and it changes into Pro difficulty.

you cannot enter cheats in any specific place... there are no proper cheats for the game, they are all just bits of info telling you how to unlock that car by racing.
i hope this may have answered your Question...
jimbob

go to your garage hold down R1 L1 R2 and L2 when you have no cars in your garage and under
2000 credits.

use action replay