Just searched every post related to editing stl files, closed vs open mesh and the general response seems to be. No. that’s not possible. CAD programs don’t do that. They don’t need to do that. Sorry.
Solidworks does it. I can import an open mesh stl file, select an edge, or select a face and work on it. I can modify the stl and then export it. Given that the vast majority of 3d objects on the net are stl format, this seems like it should be integrated instead of being “debunked” with, “No CAD programs don’t do that.” Because… they do. Day 3 of my trial, and I’m afraid I won’t be continuing. To me, this is like a photo editor insisting that a jpeg isn’t worthy of editing.