The easiest method is to simply draw a room in an elevated level, then export the room. How could I forget it, as I even used this method in the past?

Thank you, this is exactly what I was looking for!

The one nuance with CopyAsNewFurniture method - menu option not active with only room selected, so I add to selection any empty object (opening), now it is possible to use this feature.

Also after experiments I discover that floor thikness of this special level should be not less then it's elevation to get correct piece without "air" beneath

Yours, Daniel