but how does NIMH work ?
Like all rechargeable batteries, NiMH batteries can be plugged into a charger, which plugs into wall power, which recharges the batteries such that they can once again power your battery-powered things.
How can it be when my Demolisher use AA batteries ?
When you install a pack, you don't use the stock battery tray - you put a connector in your blaster and on the pack, and then mount the pack wherever it fits. Battery packs use the same connectors to connect to their charger. You can find plenty of guides on soldering and installing battery packs online. Google is your friend.
Toruk recommended NiMH sub-C, which is a common size of cell used in battery packs which is a bit smaller than a C cell (hence the name).
It sounds like you don't have much background knowledge in electronics, in which case you either need to do a lot of reading before you start working on your blaster (but don't be discouraged - electronic mods are fun!), or you'd be better off just using those aforementioned consumer rechargeable AAs.