@unknownuser said:

There is going to be some work involved whichever method you use.
This work will be far easier if the building is modelled correctly using groups, components and layers.

Thinking off the top of my head, what I would do if I received a 3d model with wall thickness is, go to plan view and turn off the roof to reveal all internal walls. I would push pull them back down to floor level, which will leave the outline on the floor. You would need to make sure there is a line drawn where the internal wall meets the outer wall so you do not push everything down. You could then just draw a few lines using the pencil tool and the inference engine to quickly add back the faces of the walls. I guess what may need some thought is how accurate you need these wall positions? If you need to create a wall with thickness as a single plane, you may need to create this plane central to the said wall.

DAVID
Spent some time on the IES site and even thought they have discussed their plan for a SU thick wall capability I am thinking that cahange is not near term for you. They make a recommendation external walls are modeled ref to out side wall location and for internal use the center line ref per your post.
Suggest you post your SU model or a portion thereof if you have some proprietary issues because how changes are made will be influenced by how the model is made. I get the idea IES wants either components or groups for their model. Assuming you have the rooms presently modeled that way changing out one model for another may not be that difficult using the SU select and replace component functionally.
Good Luck