I don't know why you wouldn't be able to with the proper adapter. Radio Crack has rc "tamiya" connector "repair" kits for cheap. Both a 9.6 and a 7.2 with the only difference being the wire guage.
I've run my FR2 off the DVD battery and my FR2LE from a 9.6v rc battery; both with the input set to 7.2v rc battery and have had no problems with either one. I already had both batteries and I'm too cheap to buy a 7.2v just so it will fit in the battery compartment. If I ever need to worry about compactness, I'll just use the supplied battery sled with either alkalines or rechargables.