Mobile Mapper Office
Is the official GIS suite of Magellan for professional mobile GPS systems. So it's able to import GIS data and convert them for mobile use onto the devices.
Here is a link with information on how to create your own maps:
The summary is: "Custom maps are generated by loading shape (shp) files into MobileMapperOffice, generating an imi file and converting this imi file to Explorist format using MMOconvertor. The imi file is loaded into the GPSr (and Mapsend Lite) as a Detailed map."
You may be able to skip step 1 if you download (OpenStreetMap) Shapefiles (unconfirmed)
- http://www.msh-tools.com extend MMO for better editing features