I have an interior. I want to use a door to teleport the player to a separate area of the interior which is closed off and inaccessible otherwise. Isn't there a type of volume or something to make it count as a separate space so that I can force NPC pathing through the door?

What? NPCs should be able to path thru teleporting doors that lead to a different teleport door in the same cell

Im not sure if thats what you want.

Nope. They won't do it. It's never worked for me. I remember in previous Bethesda games I had to set up a little volume to say "this is a separarte area, path through the door instead". Let me guess they took that out.