OK. You can do that. Those would be character classification placeables and need a PWK so the PC can't walk through the building.
I would do the following:
Extract the tile model (and textures if you won't be using the tileset) from the HAK.
Rename the model.
open the model in neverblender or nwmax or whatever and delete all the meshes but the building.
Also in neverblender or nwmax create a PWK for the model which prevents players from walking through the building.
Add the model to a new line in placeables.2da