Eureka! Well, sorta...
I finally figured a little work-around on the door gap/corridor/wall segment issue. Here's what I did:
1. Painted a prefab (I used the large storage room).
2. I painted a single, straight corridor segment outside of the prefab wall.
3. I put a door inside of the corridor segment, opening to the prefab room.
4. I selected the two cubes that included the corridor segment and the door, then went through the steps for creating your own prefab (ctrl+c, ctrl+j, save, and snapshot).
Now, I can make a new level, add my new prefab in, and then add in my wall segments for making my outdoor compound. The only thing is it looks a little weird becuase the textures are different on the wall and the first floor texture in front of the door is from the prefab floor.
I've attached a screenie.