You can add any car into traffic, as long as it is either a vanilla playable car or a mod playable car. Firstly, to find the car you want, go to Steam/steamapps/common/City Car Driving/data/physics/cars. You will find a list of all cars that you have ingame. Modded cars will often have a file with the name of the car and the creator (such as bmw 330d by modder) and vanilla cars will have a generic name like "car27". Most vanilla cars are already in traffic, so you will need to search through the folders to find one you want, as they are hard to identify. I can say that "car28" is the holden commodore, but the rest i do not know. DO NOT add car137 or car 194, as they are trailers and they will drive about with no car attached.

So, say i chose the Opel astra to add into traffic. Click on the folder and open it.
it should look like this, however for vanilla cars there may be a p_parking_setup or p_traffic_setup file. if there is already a p_traffic_setup the car is already in traffic, so no need to add it. if there is only a p_parking_setup i would not add the car, as it is not designed to be moving and will look buggy and ugly. in the file i chose, there is only p_player_setup. Copy this file and paste it. Rename this from p_player_setup-copy to p_traffic_setup. you dont need to open the file. When this is done, the folder should look like this:

To add this car into traffic, you need to go to Steam/steamapps/common/City Car Driving/data/config and find these files:here you can select the regions the cars will spawn in, so just traffic_cars is Russia, traffic_cars_eu is europe and ect. Open the file with your favorite text editing program, i just use notepad. Copy the FOLDER NAME of the car you want to add, for me its astra_by_denis154rus and do the following:
go in the traffic_cars file and scroll down until you see this list:
Go to the bottom and create a NEW LINE (dont replace or it will remove another car from traffic) and put the cars folder name in it like so:
by changing the "prob", that changes the chance of the car spawning, so a higher prob means you see the car more often.
Save the document, this should make the car apper in traffic
As we can see, the astra is now in traffic, and will appear in all the colour you can choose for yourself.

DISCLAIMER: SOME CARS ARE BUGGY AND FROM DISTANCES WILL HAVE WIERD WHEEL POSITIONS. THERE IS NOTHING I KNOW HOW TO FIX THIS, WHEN YOUR CLOSE THE CAR LOOKS FINE.