You can do this outside of Project Navigator, but be aware that tagging through Xrefs is problematic outside of PN. In my test files, I got the property data right the first time. If I had to go back and change a room number, getting the tags in the composite schedule file to update would not be automatic, at least not in ADT 2004, where these files were created. My recollection is the process improved somewhat in 2005 and I do not believe I ever tested it in 2006 to see if there were further improvements.
The attached ZIP file contains several files that demonstrate that you can do this outside of PN. The files include a simple unit plan that is externally referenced in both the first and second floor plan files, which are, in turn, externally referenced into a composite drawing, where the tags are applied and the schedule placed. The schedule is set up to scan the external references to find the doors.
In order to get unique room numbers, I added the spaces in the first and second floor plan files, not in the unit file. If you were using PN, the space could be in the unit plan, and the leveling feature could append the level of the construct in which a particular unit plan was placed.
The DoorObjects property set is attached to the door in the unit plan file, so that the location grip is active there. It is capable of reading the room numbers when xreffed into either the first or second floor file. [I modified the out-of-the-box DoorObjects:RoomNumber property to read the RoomObjects:Number property, not the RoomObjects:NumberProjectBased property, as there will not be valid data in the latter, as these files are not part of an ADT "Project".]
These files are likely not set up the way you would like, so be certain to test things out with the files done "your way" to be certain that the concept holds true under those circumstances, as well. And do test how well things update if you go back and edit a room number. You may get updates if you remove the property data overrides set up in the composite file when tagging through an xref.
Hope this helps get you started.
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David Koch
Autodesk Discussion Group Facilitator