How to create your own add-on:
>>> and read up here http://gtaforums.com/topic/816823-adding-cars-with-dlc-pack-working-template/
I just use a template (an empty dlc.rpf) and then rename the files I download.
So eg when the replacement is for the Sultan I copy the entire vehicles.meta to the folder containing the downloaded files and delete all entries execpt for the sultan. Then I rename certain links to the name I want the car to have (also spawn-name)
Look very closely at various dlc.rpf's from Add-On cars what other changes you have to maken in the content.xml and setup.xml (there you have to input how the dlc itself is named and then use that name as a reference in the dlclist.xml)
Altering carcols, carvariations etc has a steep learning curve at first, but you'll get the hang of it.
I currently have 60 custom dlc's organized per car-manufacter and/or type. So one dlc that holds all ferarri's, one for all musclecars, one for all supercars etc. After each import I test the car (does it spawn correct, is the handling correct, does the tuning work, do I like the first color is spawns with...)