Hi I see that what you are looking for or having trouble with has more here, found some more who have asked for the same so you could find some help in this, if it helps then there is a link here that shows how.
I am trying to figure out how to add addon vehicles to the server but I followed the instructions on the forum and It did not work I am trying to add vehicles but mostly police vehicles. If anyone can help me and tell me why it is not working please let me know I am willing to work things out if anyone could possibly make scripts to and possibly make custom police vehicles.
Made an example of converting a vehicle Add-On from gta5-mods to a Five Reborn resource. Instructions are in the README file and the addonr35 folder is a completed convert to use as reference. The vehicle is not mine and I linked the original mod at the top of README Support for bugs/errors will be limited