So, assuming the edges are splines, if you extract these composite curves off a face and then remove the trim from the face what are you left with?
See what we often need to do with surfaces, (sheet bodies if you like) is to effectively extend the boundaries of a face, sometimes several faces and then trim back to the original edges on a couple of sides, while adding new trim edges of our own to the areas that we're changing.
Best Regards
Hudson