No, you can only do the cash cheat one, and only one time. And for the nitrous cheat, just do this at the main menu(the screen with like quick race and challenge series and all that)(for the directions, I recommend the D-pad. That way it's sure to be the right direction.): Left, Up, Left, Down, Left, Down, Right, X
Thanks to Ares2 for the sweet sig and avy.