Is there a possibility to make skins for modded cars?

Yes, you can. You have to know the mod's skin ID, so e.g. the STOCK CAR 03 skins will be named 98F5ED_myskin.jpg. Then you can freely upload your skin to lfs.net like for the regular cars.

Thanks for the answer. That's really awesome

Of course there is one condition - the mod has to support skin textures.

Is it possible to load modcars in the LFSviewer?