Is there a door somewhere I can place in the back of a HouseLower01 model to allow the player to enter a basemant/cellar area? Here's a screen shot of the door I'm looking at:

[img]http://img560.imageshack.us/img560/6796/cellar.jpg[/img]

Instead I could probably put a trap door or something in the interior but I would think there is something I could put on this door. If there isn't a door maybe there's some kind of like portal/teleport object I could use in the editor that allows the user to activate around that spot?

Not exactly the same, but lose: BrumaExtBasemantAdd + BrumaExtBasemantDoor

I'm pretty sure there is a door, I can't remember the editor ID however. If you want to go the activator route, make a door with the mesh being the collision box mesh. You can then put that over the door to make it act like a door.

There is a basemant door like that, but it isn't exactly the same (but it is pretty damn close):
BrumaExtBasemantDoor
It's very similar and would should look okay if you position it correctly. The only drawback is that there isn't an interior version of this door as it appears to have been cut from the final game. Still, you could try using it in connection with:
SILoadLadder01

Hope this helps.