Yes this is very simle. Have a look in what directory the circuit
files of the track pack are. The circuits of gp2 are in the directory
gp2\circuits. The circuits are numbered: f1ct01.dat is Brasil,
f1ct02.dat is Pacific and so on.
Now you can simply exchange as many files you want for example with a
batch file. Only change the number of the file to what ever circuit
you want and replace only one or two races with the new circuits.
There is only one problem: In the menue of GP2 is still the name of
the original circuit because it doesn't "know" there is a new file in
the circuits directory.
I made a batch file including some new "french" adverti***ts for
Paul Ricard. Its simple to switch between the tracks without loosing
the original files.
If you want I could mail you a copy of it.
Regards
Armin