It just can't be done, I tried with level templates, copy pasting between levels, between projects. The last attempts just crash the editor.
I wish they made an easy export, import function. As it stands now you cannot make a library of houses or anything that uses walls, nor can you edit objects once you placed them cause those changes are not taken into prefabs.
Another major disappointment for me

Sword Coast Legends + Larian Terrain Editor + NWN interior editor
Then we have a GM worthy game, can someone PLEASE make this ...