You can buy supplies at e.g. Radioshack to
make a home-made battery pack using D
batteries, and connect it to the supplied
battery pack. This will give you more time,
but just more hours, not days. I don't know
about piggyback multiple battery packs
though.
An easy (though not cheap) way to gain a
significant time advantage might be to buy
the large portable powersources that you can
plug household appliances to e.g. when you
are camping. They can run desklamps,
computers, etc, surely they could keep a
little camera going for awhile. These
things are basically just really big
rechargeable batteries with 110 outlets
built in. Of course they are very bulky,
and you would need to have them somewhere
they are easily accessible to you to
recharge, but not easy for someone else to
steal.