Sadly, I tried this, and it didn't work. I noticed that this post is from 2016. Perhaps EA patched something to make this not possible anymore?
@FayesCannon You can upload apartment as one room or as a room to the gallery by placing a wall in the room but dot use the arrow
This is were you'll need to make a wall first either in the room or on the outside on its own and pick it up
If you have the wall inside the room pick up the wall to place it no arrow to be use if you do it will make a separate room
Now it's one room with in a room and always save it to your Library to make sure and place it either in the apartment or on a Lot as one room before you share it to the gallery
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@FayesCannon You can upload apartment as one room or as a room to the gallery by placing a wall in the room but dot use the arrow