This could be done. 90% of it could probably be done with JSON only edits. But if you want this done, you’re going to have to do it.
Start by defining the new frames and other parts for the MODERN_CAR. Then add new modern vehicles using the new parts. Then add those new vehicles to the vehicle spawn lists.
See Guide to adding new content to CDDA for first time modders for advice on how to make all this happen.