If you decide to use a building placeable in your module, you will need to use a door placeable along with it, and fiddle with scripts to get your transition working. Or so I thought until rolo_kipp reminded me of an old trick. if you want to have a proper door, with area transition, for your placeable building, just do the following:
1) Place a building tile near where your placeable building is.
2) Place a standard door in the entry of the building tile.
3) Find that door in the asset list on the left-hand side of the screen, under map name, Doors.
4) Right click on the door in the asset list, and choose Adjust Location.
5) Fiddle with the X, Y, and Z co-ordinates, and apply, until the door from the tile is in the doorframe of your placeable buidling.
6) Delete the tiled building, and enjoy your new door, complete with proper transitions, in your placeable building.