It's possible to race any car on any track on XBOX LIVE. Follow these steps:
1. Create a session.
2. Go to "settings".
3. Select the series with the car you want to race (ex: DTM).
4. Press A to select that series.
5. Press "continue".
6. Go back to "settings".
7. Select the series with the tracks you want to race (ex: StockCar).
8. Press A to select that series.
9. Do NOT press continue. Go back into the series selection screen.
10. Select the series you had previously (ex: DTM).
11. Press A.
12. Press "continue".
13. Go to "track"
14. Press "X" to "reset championship"
15. Go to "track"
16. Select the track you want from those listed (ex: Texas)
And voila, you have any car at any track. In this case, you'd end up racing DTM cars at Texas