Yes you can.
I own (or have owned) two driving games. Turbo and SCI. Both of them had shifters that activated microswitches (interface like you would a standard button). The steering wheels were both interfaced the same way as spinners (I think it is an Optipac you would need for that).
Gas pedal could be different. The gas pedal on my turbo seems to be 3 position optical (I say optical, because it doesn't seem to actually touch anything, and doesn't work if you are shining a flashlight at the assembly). Other games have gas pedals with encoders, microswitches, or pots.
Either way, you should be able to interface all of them. Some steering wheels use POTS instead of an encoder wheel. Those can be intefaced through the joystick port. (Search for something about a Star Wars yoke hack, same way for those steering wheels).
Also, new driving controls from Happ cost a fortune. Get some used ones, or buy a dead driving game at an auction. It will be alot cheaper that way.