So after I import .map into UE4, I can only export one brush at a time? 
Ouch…
For PC it doesn’t really matter. However, I began thinking about mobile VR and since number of drawcalls/tris is quite limited on mobile VR, the best way is to keep each area on the level as one static mesh, with one material and one diffuse texture. If you manage to make use of .proc file for doom 3, then only visible faces could be imported into UE4. And if I could export whole map as FBX, then I could simply bring it to Blender, duplicate it, join all copied meshes into one, slice it up and the bake textures from originally imported geometry to the slices. Re-export as FBX with only so many pieces and bring it back to UE4. This way I can satisfy mobile VR limits. I know it sounds messy, but that’s just reality for mobile VR at this time.