you certainly want to upgrade Global Mapper to the most recent version, to quote Mike at Global Mapper on the yahoo mailing list
" The latest release of Global Mapper should include support for
generating custom sized .TER files. We also have a build available that adds
world and PRJ file export for Terragen .TER export if that is useful to you."
Add that to the ability to easily export properly format and georeferenced World Wind format tiles for imagery, which one could whip up with with orthographic renders in TG, and you have the ability to texture the whole globe( if you have the time and disk space) for 3d viewing in World Wind.
Here is a link to some testing we have done at
me-dem.org using custom imagery overlaid on custom terrain of Middle Earth's Anduin Vale,,
http://www.me-dem.org/component/option,com_fireboard/Itemid,43/func,view/id,2396/catid,8/ and this is a link to a blog I have on the easy way (so far) to get custom terrain, as in terrain you create in TG , GeoControl,L3DT, WM and so on in to World Wind
http://wwterraintiling.spaces.live.com Imagine a complete fantasy world heightfield custom textured inplace of the Earth in World Wind, in time it could be done. SeerBlue
if the idea of custom texturing the World Wind Globe, even a small section, pushes you over the edge and you go for upgrading Global Mapper, tell Mike me-dem sent you