It seems that one good way to integrate a house that is drawn in Sketchup with a base/terrain drawn in FormZ is to export the base as a 3ds and import it into Sketchup. The key is to use formZ to mark where the footprint of where the house goes.
Export the object(s) that intersect the ground plane from sketchup as 3ds or dwg. Import them into formz, integrate the house objects with the terrain mesh, then difference all or part of the house objects from the terrain. All you really need is one corner of the house to be differenced out of the base.