If you are a skin maker and have some skins, that are better then original, can be integrated in a multiplayer - sure... We do allow some modifications of skins...
We do not allow the cars modifications and texture modifications (of cos cheats, mods, that give advantage over other players too)...
If you made a skin by yourself and want players to see it and use - you can always send the screenshot of skins, you made, and we can post it on the web in section "Downloads" with the name of creator...