Our ReVolt Pub

how do you make carboxes?!

If you mean the boxes from the Front End selection screen, then you can't.
You can replace the graphics of the specific bitmap that corresponds to a specific car, then replace that car with yours, just so you get the effect that your car has it's own box, but this means rplacing stock cars.

Does replacing stock up the game in any way?

If you mean mess up the game, then no.
As long as you place your cars in the same folder names as one of the originals.

is there any program or tutorial to make the car boxes like inthe game?