You'll have a hard time, unless there are some new tricks I don't know, trying to smooth out those surfaces.
There are a lot of cars available in native ArchiCAD format, and a lot of them are free. Unless you need very specific models you can probably find ones that will work for you. You can start with the
object_depository here on the talk. Go to "02 Sitework" there you will see the "Street & Traffic" library which has some nice vehicles.