You are almost there.
Do what you have done with slabs and then save this selection as an object.
Or better still if it is to be a door save it as a "door" object and then it can be used with the door tool and will cut the hole in the wall automatically.
There are many posts here and in Archicadwiki about saving as objects and door/window objects.
Barry.
One of the forum moderators.
Versions 6.5 to 27
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