You actually can build your own championships, even with mods.
Just get the CTDP championship manager nx (google is your friend), which was originally created for rfactor. Since the engine is the same or similar, it will also work with GSCE. Once you downloaded the championship manager and installed it, you will be asked to point to the rfactor.exe (because it was build for rfactor). Now go to your GSCE directory (normally d:\Programme\Steam\SteamApps\common\Stock Car Extreme), make a copy (!) of the GSC.exe and rename it to rfactor.exe. That's the magic. The championship manager will now recognize the GSCE directory and you can build your own championships.
PS: I don't know if that's legal...if not, please remove it @staff
EDIT
In order to find your custom championship under Championship in the main menu, you have to select the specific car/series in the main screen. If you have selected All cars and series you won't find it.