You can import a map terrain in an IfcSite in BlenderBim like any other object. You must create your terrain with BlenderGis (I use a "*.asc" file to create the mesh and add a orthophoto to texture in blender ). This mesh can be added to the ifc project like a building element proxy without problems. If the terrain is big, when you export this element to ifc format you get a big ifc file with the mesh (and losses the texture, or at least I don't know how to export it successfully).
You can see a sample (terrain about 15 km long) imported with BlenderGis and textured with orthophoto in this thread
I might be far too late and irrelevant, but FYI. I made a short video on that subject (in french but i'm sure it's quite understandable without sound) -
Generation a terrain "leaf " in Blender3 BlenderGIS and copy/ paste to Blender4/BlenderBIM before assigning to the IfcGeographicElement Class. It seems to work fine.
Yesterday was a cold day, and i published this video that involves use of QGis and QGiq2ThreeJS to get a Terrain model usable in Blender and BlenderBIM. If you have access to DEM and relevant GIS vector data, you may use this method, it's a bit longer than the BlenderGIS method but i usually use this kind of process to have more precise data on smaller areas.