Yep.
Open the gp4tweak_data.ini in your GP4 main directory.
Under [CARS_NUMBERS_PER_TRACK] you'll find following line:
[Track]= 1, 1, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 1, 1, 1, 1, 0, 0
This line difines which cars (by number: first Schumacher, second Barrichello, ...) will start in the race of the appropriate track.
Change this line to
[Track]= 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 0, 0
and you'll have 20 cars in the race.
The tracknames are from 2001, so in this case I estimate "Nürburgring" will be the right one because it's the 9th track and Indy was the 9th race this year.