Getting power out with a fully charged battery is the easy part! The hard part is the reverse.
Take a look at this device: [guide|19678] as you can see theres more to it! This device allows you to charge the battery and then run a device off of the battery (two connectors). You would need to copy all of this to make a workable external battery.
How about finding an external battery unit which has a bad battery and fix it up? You'll need to find the same battery the pack has though.