Might find it difficult to get a suitable DC transformer as you need 7.4V ,
5, 6 & 9V are easy to find 6V might work, you might get away with a 9V and use 2 1N4001 type diodes in series
with the +ve lead but it;s tricky, and it also means your camera isn't excalty portable any more
as you'd need to be near a power socket.
The E500 is a little old now is it really worth the effort ?
I'd just buy a new battery if I wanted to use the camera for a few more years.