I thought I had read that the career experience bar goes up anytime a B-Spec race is held while the driver is offline - even if he is not in the race at all. If true, that would mean that a driver could fill his bar with substantially less than 500 races in his stats. But you'll still have to run the 500(or however too many you have to run) anyway, it will just be his profile that shows different.
If that is the case... I wonder if it would be possible to get the trophy with a driver who's never driven a race at all?
This is probably the most definitive thread on the subject.