I don't know about that particular sim, but very old games (GPL and RBR) have third-party plug ins, new games have built-in support and those in between require you to edit an ini file, for example GTR2 requires you to change a line about holding a button to select gear. You probably know all this, but a quick google search revealed no plug ins, so I'd go digging around to see if there's anything in any ini files that look relevant. Problem with that is that might mean you have to use the shifter with all cars, and in fact if there's no mappings for individual gears then there's probably no way to do it without some sort of mod.
So in summary: I haven't got a clue. You might be able to find useful help on the nogripracing.com forum, Sim Race Mirror Zone (though they seem to deal almost exclusively with GPL) and other more general sim racing sites.
