I find it easiest to import the entire area with KOTORMax's area tools. That way you can see the whole thing and not have to hunt down each room, and you can still export a room individually by selecting its base when you're done.
It's not super complicated, but the engine is very particular about what it wants.
Only one walkmesh per room.
The walkmesh must be a trimesh (not an editable poly or any other type of object). Safest thing is to apply an Edit Mesh modifier or convert to editable mesh when you're done editing.
The walkmesh can't be self-intersecting or have any gaps. Both of these lead to unpredictable issues. So it's best to work from one mesh, extruding, cutting, or creating polys where necessary, and not build it with different objects and try to merge them later.
The walkmesh uses a special material to configure what areas are walkable, with a sub-material for the different surface types.
In this case, there's already walkmesh where the beds are. That part of the floor is set to non-walkable:
You want to chop up that pink rectangle up so it has a walkable surface covering the bed. Working with trimeshes can be a pain, so I prefer to add an Edit Poly modifier first and remove unwanted edges like so:
Once the part we want to edit is actually a real rectangle, editing the geometry is a lot easier.
Just make sure to make it a mesh again when you're done.