I have asked this question a long time ago, and unfortunatly never got an answer. The only solution I could come up with, was to stay offline, delete all the computer cars, and just make up my own stuff.
Failing that, convert the cars into GTR2, and then you can start making up better complexes for championships etc.
Sorry, but it's not the best solution, unless SimBin make it work where you can race just your custom skins.
HINT HINT SIMBIN if you see this!!!!!