NPC's can be shared/re-used in any zone i.e certain bats can be found everywhere. So what you need to do is go into spawneditor and add that npc into the spawngroup, so yes you can but not directly in npc editor.
I think your question is valid, however I think adding that functionality is complex and may yield errors in the database.
I'm off now to make the spawneditor ready for 0.7.0-800 06/07/2006 build date.
-edit - now the spawneditor should be 0.7.0-800 compatible. A few feature enhancements such as error checking is added .
You can now type in the npcid and the program responds with npcname automatically.
Also, clicking on the npc row (in spawnentry grid) will populate the NPC_search pop-up and you can thereby search for the npc that way.
http://66.159.225.58/eqemu/GeorgeS_EqEmu_Files.zip
GeorgeS