As mentioned you need to get an appropriate dropin (voltage range suited to your needs). There are heaps around.
Like Think2x said 2aa is 3cr123. This is probably the easiest mod.
FM has made specific aa hosts. He also has an 18500 body that would be perfect for 1aa.
Dereelight and Solarforce have specific tubes as well.
If you want to use a 2cr123 light for aa you will only get one battery to fit, but still a relatively easy mod. Just like think2x mentioned find something to roll up the same length as the battery tube. Find a metal spacer half the length of a cr123 cell to complete the circuit. Use you imagination. I have used cut down bolts etc and molded silastic around them to ensure a good fit. Or just wrap them in tape. I have some ball bearings that came from 4 sevens for charging rcr123 in an ultrafire charger - these do the job perfectly.
Or you could make up a sleeve/adapter that allows you to use a single aa or a single cr123 without carrying any extra parts around. I did post some pics of one I made up a couple of years back if you can find it. It worked with a 2cr123 light.
Obviously with a single alkaline/Nimh aa you will not get nearly the output you are used to with your cr123's etc.
Maybe sell you current lights and start again with lights more suited to your chosen battery chemistry