When you import a DWF file as a Space Plan, I'm pretty sure that FMDesktop strips out any fills associated with rooms in a DWF file created by Revit. You may notice that it strips out the room labels from Revit as well. This is necessary to "make room" for the fills and hatches that are added dynamically by FMDesktop. The room labels are removed to make room for the dynamically generated labels created from the FMDesktop database.
I haven't tried importing a Revit DWF with fill patterns as a Plan View in FMDesktop. So I don't know whether they are removed in that process. I could see leaving them intact in that case.
Mark Evans
Autodesk
Mark Evans
Senior Product Manager
AEC Division, Simulation Product Line
Autodesk, Inc.