Sure, it's tough and annoying task but I'll try explain how.
First you'll need modeling program (I use Blender for example) and a car file to open on Blender, then once you have it on Blender you'll need to test it; to see if parts are stuck together, if it fits on the center line, and lots of other messing around too. Once that's done you'll need the '3DT export' python code which the admin made, it's on some of the S-tuner Facebook groups, not to hard to find. If not I can PM you the code. Then once it is in .3dt format you can import it like a saved car on S-tuner, then you'll need to test it on there, adjust on Blender again if needed. Aaaaaaaaand you'll have your car imported onto it! Wheew, I hope that helps.