Yes, but only if you create a custom championship I believe (unless anyone knows another way).
NOTE I have not done this with this latest update, so I cannot guarantee it will work, but I see no reason why not.
1) use FerrariMan's custom championship creator to build your own season e.g 'multimini test'.
2) once done, you will find your new season file 'multimini test' that the above program has created, in the rfm folder.
3) open this file with notepad and add the line
max opponents = 50
4) now you need to go to the vehicles/mini_challenge folder, and copy some of the existing minis. For example, copy the BMR folder and rename it to BMR2, then inside change all the BMR26 files to BMR28
5)In the BMR28.veh file, change the number from 26 to 28
6) do the same for BMR27, changing it to BMR29
You have now added two cars.
7) Repeat all the above as many times as you like.
NOTE - of course what you will have done is add duplicate cars on the grid, unless you also replace the skins for the cars you have added.