I know you can use various addins (like Case Extrude Rooms to 3D Mass) to make a form out of a Room element in Revit. However, there is a vanilla Autodesk way you could go about this:
- Export a Revit view to Navisworks with all of the Room-related export boxes ticked in the Navisworks Exporter addin
- Open / append the NWC in Navisworks
- Turn off everything except the Room objects that the Navisworks Exporter created
- Export the scene to FBX, convert to DWG, import it to a family, place that family back in the project (using parts of this workflow). Use origin-to-origin linking to get things right.