This is from google. Just did it in case your results were constrained somehow... selecting objects in the tree (#4) is a key point...


1. Open Draft Workbench: View->Workbench->Draft.

2. Create a new file.

3.Import your DXF: File->Import.

4. Select all necesarry objects in Tree view (you can use Ctrl or Shift to select many objects)

5. Use a tool Draft->Draft to Sketch.

you have a new, automatically constrained sketch (see Tree view)