Hi,
You cannot modify the navigation control hosted within ArcGIS Explorer. You can only change its position and enable/disable it (available under Explorer options > Map Window). You cannot host any other controls inside the map window. I can also confirm that there are no current plans to provide this capability.
However, using the SDK you could write an Add-in that controls navigation. The MapDisplay class contains several members for navigating the map/globe (e.g MapDisplay.ZoomTo method)
Mike Rudden
ESRI
