Just to extend from Oolite, you will need to consider the backface culling (otherwise in DBP it will look weird) you can basically cut the door holes etc. out, however, there are better ways of doing it, using 3DWS would be an easy route but it may not be worth the money, so you can stick with blender, treat the walls as box, resize them, skew them, do what you like until it looks like a room and texture them.
However, I would suggest a certian tutorial by Megaton Cat (I think its in DBP talk) that is based on level building, its not directed at Blender, but the knowledge is all there.
Quote: "Your extensive knowledge of level design blows my mind."
Oolite is actually a skilled modeller, however he was a bit vague