yeah, there is such a editor that allows us to do such a thing, it's Carset Creator 4 made by Dummy (dumbass)
but that editor is not released yet, due to some...er...i don't know, better ask him yourself ;P
about your cars.wad file...simply make a backup of your original or tweaked carset...
e.g renaming them cars.wad.back
or simply copy that file into another folder...
that's what i normally do, create a backup file for all my wad, dat file