Step 7️⃣:AddDATAfrom the new car(if the car you want to put as Add-On didn't have data when you downloaded it, take data from the original game car that it replaces)to "vehicles.meta", "carcols.meta", carvariations.meta" and "handling.meta", which located in "/FOLDERNAME/dl...
I mean this is the franchise where almost everyone of us went to just to slaughter people and get into police chases when we were young and still it's a nice time in single player but in online it might be fun for you but not the person at the other end of the stick....