Here's how I've done it (using those great cars that you've created too). Take the simplex robot for example. Open the simplex XML file (in data/drivers/simplix) with a text editor. You'll see the default cars in a list, each "section" with a number (starts with "section name" and ends with "/section".
Easiest thing to do would be to copy the last "section" and paste it at the end of the XML file. Then, change the number to the "section name" (add 1 to it). Change the "name" (make something up for your "driver". Adjust the "short name" accordingly. Adjust the "code name" accordingly as well (this is the 3 character abbreviation that appears in the side scrolling ribbon of race standings).
Now, here's where you add the car. Adjust the "desc" to match the car that you want to add. And, adjust the race number, just give a number.
Here's an example using the TA50-deckard-conejo67:
<attstr name="name" val="Mark Donashoe"></attstr>
<attstr name="short name" val="M. Donashoe"></attstr>
<attstr name="code name" val="DON"></attstr>
<attstr name="desc" val="simplix: car1"></attstr>
<attstr name="team" val="simplix"></attstr>
<attstr name="author" val="Wolf-Dieter Beelitz"></attstr>
<attstr name="car name" val="TA50-deckard-conejo67">
<attnum name="race number" val="16"></attnum>
<attnum name="red" val="1.0"></attnum>
<attnum name="green" val="0.3"></attnum>
<attnum name="blue" val="0.3"></attnum>
Then, in the robot directory, in this case, simpiix, add two folders. One with the "number" of the section name (so, if you had 12 cars when you started, and added a 13th car, you would have a folder named "13". Add another folder with the name of the car that you have added, in the example above TA50-deckard-conejo67
Save the file, replace your existing simplix XML file.
Then I've copied the "default" file from one of the existing car directories in the robot directory, put i into the car directory that I just created. And the copy the wheel, preview, "paint", and shadow files into the "number" directory that you just created from the corresponding car directory in data/cars/models.
It works for me. If you'd like, I can send you a copy of my edited simplix XML to take a look at. It appears that there is a limit on the simplix robot of 100 "drivers" or cars. I've noticed a limit on other bots to 20 or less.
Hope this helps.