You can import a dxf into a drawing and then delete idividual lines. I usually set up a variable drawing format that is the same height as the part I am making and import the dxt to fit the page. You can copy and paste imported drawing entities to different sheets also.
Once you have your sketch, you can export an iges (as splines) file that you can import back into a part file. Remember that the lower left hand corner of the drawing will be you 0,0,0 point when you import the sketch into a part, so you may want to move the sketch entities to that corner.
You can sketch over or use the imported geometry. I usually sketch over it (with out making a dependency) because something will change.
There are many ways to import and export files. I'm not sure what is the best way. I usually get different results depending on where the dwg/dxf file came from.
Hope this helps.
survey