How to trace an extrusion cross section.
Using an existing IGES or Step file of an aluminium extrusion, create a complementary part; i.e. an end cap or piece to slide in the extrusion slots.
not 100% sure of your question as im not familiar with terminology being mainly self taught however going to take a guess that this will help
after u import yout iges or step file into freecad if u rotate it to the face u want to trace is in the upward direction u can create a new sketch in the xy face and draw the sketch tracing the original object ... hope this helps