Currently, this functionality doesn't seem to exist. in xDesign but xSheetmetal DOES have the ability to extract a DXF from a selected face. When I had trial access to xSheetmetal last year, I discovered that you could open an ordinary xDesign model (non-sheetmetal) in the xSheetmetal app and the DXF extraction function would work on those models also. I think that adding DXF export for faces and sketches from xDesign would be a handy addition to its capabilities.