This Russian site has all the details needed to upload new maps to a Magellan eXplorist 300, 200, or 100. Inside the battery case are four pads that can be used for serial communication. You will have to use a MAX232 to shift the voltage levels. Since this occupies the battery space you'll need to provide power as well. This can be tapped from a USB port. Once the cable is built you can download waypoints using OziExplorer.
[thanks Chris, Cary for corretion]
http://www.hackaday.com/2005/12/27/magellan-explorist-upgrade/