How to enable add on cars for offline server?

I made a copy of the entire "data" directory used by one of the online servers, that includes some additional cars. I can see the add on cars are there using vlted (editor), and I can launch the offline server (I'm testing with 1.7.0), but I don't see the add on cars in the car dealer.

I assume I need to edit some file in the offline server to get this to work?

From what I can gather, you need to add basket data and edit the catalogue file. Unfortunately I don't have specifics and sadly there is hardly any documentaion. I'm trying to make some edits myself and have to mostly rely on trial and error.

you can also edit the car hash in your cars file and make it spwan in your garage