Is it possible to switch 6 1.5V AA batteries to a Car battery with a 12v-9v converter to get a longer lifetime out of my Gigapan Epic?

Theoretically yes, with the caveat that unless the device has an external power input you may be voiding your warranty by attempting this (if it's got an external power input then it's probably designed for running off of a cigarette lighter power adapator, which is essentially the same thing as you're describing). In practice I'd recommend using a marine battery instead of a car battery. (A "marine battery" or "deep cycle battery" is designed to be repeatedly charged and fully discharged; car batteries tend to develop problems if not kept pretty near full capacity most of the time. You may need to ask in the sporting goods department instead of the automotive department to find someone who knows what the difference is and can point you to the proper kind of battery.)