There is a difference between user added content and modying existing content. While in BeamNG (and AC,rF) users add extra content,in LFS it's not supported yet. Modifying existing model breaks up compatibility online,which is practically the main idea of LFS - it's online racing simulator as you know.
And for those who will come to swear on their mother's grave about vob mods being compatible online - some might work,but still it messes up everything - for example skins do not fit if you have a different model,while having different car model sizes cause ugly effects - like cars "melting" together if the replacement model is larger.
Scawen already posted that he intends to open LFS for user content,but that will happen when he considers LFS ready for that. Until then stay "underground" or use other modding simulators.