Long time ago in RHino 3 when you were exporting models from Rhino as dwg. file you had an preview button to see how your meshes would look when exported. I dont see this in Rhino 5 
Any chance you can preview how you model will be meshed up in RHino?
Hi Mario- probably the way out is to use the Mesh command to create meshes first, then Export these to dxf.
-Pascal
Thanx Pascal!
Thats the one!