We work with Autocad also, but people make "hatching"/fills there, so it doesn't count/calculate the areas in meters square. Can you make converter from hatches into zones?
And by the way, converter from fills into walls will also be good idea. convert fills into floor/ceiling. Converter of "hatching"/fills into roofs. etc.
Main idea is, if Archicad converts from some 3d object into 2d, and additional info is lost, there should be a way to convert that back (semi)-automatically to 3d objects. Well, there should be some conditionas. Especially for walls. To convert automatically from rectangle into a wall, this rectabgle must have certain ratio of width/height (though, some walls may look like pillar). Convert "hatching"/fills into a pillar etc.
You cannot get back information that did not exist... Reading an AutoCAD hatch boundary and applying it to a Zone might work, but for Walls, Slabs and Roofs, there are too many variables that you would end up needing to check everything if you could get it to correctly determine what is what in the first place.
For Slabs and Roofs, if you merge the .dwg into the model, can you magic wand them if they are complicated? Right off the bat, you will get all your correct thicknesses, composites, angles, etc.