I just did this with the new endurance cars. Here is what I did in a brief outline.
There are a bunch of steps and I could have done things I did not need to do, but I'll report here what worked for me.
First I used the Mas editor and extracted the .veh and .dds files from the supplied Oreca car-upgrade.mas. I extracted these to a work folder.
I opened each dds file in The Gimp and changed the color of the car with the paint bucket tool and used the text tool to replace the stock numbers with my own. I created two dds files for each of the stock cars (seven were provided for the orecas). Each of the dds files were named Oreca#xx-alt.dds with xx being the number of the car.
I then made a copy of each of the seven veh files. This gave me 14 veh files for the 14 drivers I was creating. I opened each veh file and changed the name, car number, team name and dds file.
At this point, my work folder contained 14 dds files and 14 veh files. I created a new mas file with the same name as the one provided (car-update.mas and was version 1.09). I then created a single component in the mas editor and pointed it to my new car-update.mas. I gave it version 1.10 and told it to update version 1.09. I built the file and installed it from the mas editor. At this point in the RF2 launcher in the package section, I could see my version installed along with the 1.09 version. Now, when I run these cars and go to the Session Setup screen and select 13 AI, RF2 will select the new cars I created (plus one is mine to equal 14). In the Choose Car screen, I now see only my created drivers in the list. Note that if I choose 15 AI in the Session Setup screen, it will grab a fictitious name for that one.
Now I wanted to set up a separate Series. So I did the standard series setup process in the mas editor. I created a new package, chose my tracks. When I went to select the cars, the new cars I set up showed up in the list. I believe it does this because I updated the original version to my 1.10 version so it knew those cars existed.
I am going to say something now that I THINK is true, but I'm not sure. When you choose the cars while building a package, you can check the boxes for each car in the right pane. When you do this, I believe it loads those cars directly into the package. If you don't add them individually, I think the game calls them indirectly. I THINK this matters if the cars get an official update. I BELIEVE if you load the cars directly, they will stay as part of the package and will not be automatically be replaced by the new updated cars.
Once everything was to my liking, I built my series package and installed it from the Mas editor. When I go to the Series, it loads my new cars, and I'm happy.
I named all my AI drivers after famous authors, by the way, so I'm racing against Hunter S Thompson and James Joyce, et.al.
For completeness, I also did the same thing to the Normas (new veh and dds files) and I named them after American presidents. I added them to the series and I also added the Porsches to the series although I didn't mess with them...they are just racing fodder.
So in the end, I have a series where I have 14 Orecas driven by authors, 12 Normas driven by U.S. presidents and six Porsches driven by generic named drivers. My session setup says to use 31 AI (remember to subtract your car from the total).