A popular steering wheel controller for race fans seems to be the Logitech G27, it is a top-end (£250+) controller that features force feedback, paddle shifters, hand stitched leather and 16 programmable buttons. The problem is that only 6 of these buttons are on the wheel itself, imgur user benavery managed to solve this problem by adding 6 extra buttons and a toggle switch to the wheel.
Using a perspex sheet to mount all of the buttons he covered it with some eBay faux Carbon Fiber adheisive sheet. This turned out better than expected and the buttons really do look stock. The electronics are simple, a craigslist find USB joystick was used and new switches wired up to its contacts. This then just connects to the PC and is picked up as a second controller allowing him to program each of the buttons to do his bidding.
If you want to see more photos of the build process then you can check out the imgur page here http://imgur.com/a/mLqa0#DT0i2gI