We have not started to use smart walls yet and I'm working on a project started in V11 with single line walls. Is there a way to insert a smart door (or any door - similar to the door command in V11) into this type of wall using V12?
Sorry, but a smart only cuts into a smart wall.
If you have old walls, you can still cut the old doors into that wall, just turn off Auto3D in the Architect Menu.
You an place a smart door anywhere if you do not have "cut wall" on but then you need to define the jamb wall width.