What do you mean exactly?
Maybe you want all 2006 and all 2007 cars race togheter?
If so, edit the file SIM_WTCC.gdb, you should see two lines like these (I don't remember the text exactly now....):
WTCC_2006 = WTCC 2006
WTCC_2007 = WTCC 2007
Well, add another line like this:
WTCC = All WTCC cars
Then restart GTR2 and go to the panel where you can select a car.
You should have different categories (Official, Single Makes, Specials, WTCC,....).
Select WTCC and you should see three options.
Select All WTCC Cars and you will have all cars togheter.