It turns out the process is marvellously easy. All cells are connected by doors, entirely standard doors, which have been set by an option in their Properties window to teleport the player to another door in another cell. So just a quick note of the relevant door's unique ID# from one level, input into the teleport target field for the other level's connecting door and essentially... bingo! Though the arrival point and direction have to be slightly fine tuned.
However this wasn't the goal of the day. Recreating the layout of the dungeon known as Khalith's Cell was, so I began with that. A little experimentation and thought showed the Vault (Rusted) tileset was limiting and inappropriate, so I switched to (mostly) the Utility tileset, with a smattering of Underground and Cave.
Toilets were the most diffi... oh, crap, I just noted some missing small rooms on the West side of Level 2. I'll make a note to fix that tomorrow. Anyway, toilets. Using the UtlRm (Utility Room) series didn't seem right. All those thick pipes running vertically in the corners and tubes looping around the ceiling? What toilet has stuff like that? Apparently the ones in Fallout 3's utility tunnels, judging from a little research in Fort Bannister.
At this early stage there are already things I see that need fixing. Following the original plans to the letter may have been a little reckless. There are far too many long, straight and at least so far featureless corridors and stairways. Even looking forward to their cluttered and combat-prepared state, I can't see them being much fun without serious adjustment. Similarly, there are two large chambers - one on level 2 and one on level 3 - which take a long time to cross and suffer the same problems.
Anyhow... tomorrow, I think I will be adjusting the layout in the mentioned places and adding in ambient testing lighting, the lack of which you can blame for the absence of any actual screenshots.