I write this in English as i hope to reach the feedback of most players.
i also hope i express myself well enough 
Ok, so we already have i. E. Chairs ingame. We can sit on them. That's an already working mechanic.
I would like though to design my own chair. I can do that with pnb but sitting on it doesn't work as the game doesn't recognize it as a chair.
I could of course place a regular chair and make pnb overlap the original one. But that's just a workaround.
My suggestion is to have a ghost shape of a chair that is giving the frame for it. When using pnb we would have to build the chair within this premade frame. Only stuff that is placed within the borders of this frame will be recognized as chair.
When done we would have to "finish" or masterpiece somehow, so the game transfers everything that was properly placed within the frame into an object (chair).
The frame would prevent we build a carshaped object and call it chair. And it would ensure the game won't have any collision or interaction problems.
Once the masterpiece is labeled as finished we couldn't treat it as pnb anymore but only as the object that we originally chose.
Chairs are just an example of course. Could be also doors, grill, etc.
