I am not sure that I would go to the trouble of creating a whole new second file.
I would either drag a copy of the door (and wall) to another location or even another 'special' storey so it is still within the original job file or I would select in plan and save as a 'Module' (.MOD) file that can be merged back in at a later time if necessary.
A MOD file will only contain what was selected when saved so it is a bit like copying and pasting from another job without having to open that job and select the bits you want.
Barry.
One of the forum moderators.
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