I appreciate that this is an old thread, but just in case somebody else is looking for a similar solution:
You can easily
(< click the link to see a demo video).
Personally, I think it's better to charge the batteries separately and outside of the unit whenever possible. All decent NiMH chargers will charge each battery on its own, monitoring its voltage and temperature so that each cell gets the right amount of charge - the Garmin GPS doesn't do this, it charges the two batteries in series rather than individually, which will ultimately shorten their life span.