Hey there,
yes, you can pre-place any building and it will work. Depending on whether you want it to be saved with save games, you can also enable “PrePlaced”(which will disable dynamic savegame save/loading, so that it is not spawned by savegames).
For floors/walls, you can do this trick:
First, play and place your floors/walls like you want to.
Then press this button (F8) To get into editor mode and select a floor/wall you want to copy.
Search for the instanced static mesh and copy the instances.
Now, in your preset level, drag BP_Floor into the game and paste the list of instances into it!
This way, you have a pre-placed layout!
Remember to set the building’s position to 0, as all instanced meshes are saved in world position. Also you need to ensure the Floor IDs are set correctly (0 for ground level, 1 for the one above etc).
Also, each floor has 1 floor actor and 1 wall actor. So if you prebuild 2 floors, you need 2 floor actors with floorid 0 and 1 respectively. Each of them has a single InstancedStaticMeshComponent to save on performance. (You can easily adjust this logic in the RestaurantManager though)
This is actually a good feature to introduce within the editor to make this process easier. I will look into adding this 
Cheers
Alex