I've seen pictures of someone that mounted a post on the steering wheel of a surface transmitter that allowed him to steer with his thumb on the end of the post.

But the easiest would be as Ted mentioned, which is using a stick controller with steering and throttle controlled by one stick. My stick controller has the throttle with a non-spring pot, meaning it holds it's position once it's moved. But I don't know if that would work since driving a car, other than at a low speed for someone that's used to driving, requires constant throttle adjustment.