Legoing together 6 volts worth of battery to the camera and getting it to operate is pretty easy and straightforward. 5 x nimh is ~6v.
The problem is properly charging that pack once you make it. The nicad charger that came with your camera doesn't have enough umph to properly juice up the nimh
Lithium ion would be a pretty sweet pack to make. You could charge it very quickly. But it needs to be balance charged with a purpose-built lithium ion charger, and you're going to need to do some kind of voltage regulation because a two cell pack is going to be way more than 6 volts.