Have you guys talked to the Hexagon 'gis' guy about this? I've just been sitting in the first two map build classes recently, so I can't really answer your question. Seems like a reoccurring theme that needs to be dealt with....
Edited a little later: in the classes I've sat through, the instructor mentioned using outside map sources for visualization purposes. For CAD functionality you have to process the data into the Hexagon format: a process not unlike the orginal Spillman outfiles or the Versaterm VGI tool. At any rate, would consuming an ArcGis server map or AGOL etc be a viable option in addition to the CAD map?
That should just about do it....