Update: I got Designer. I can also open my project in Designer and paint the line in right there. There was some weirdness going on, as changes from one App did not show up in the other right away (after saving to iPad). I think I needed to closed and reopen the projects. I also got worried and created a bunch of copies in between - so Im not sure what the most efficient workflow is yet. But, bottom line: I was able to open my file in Designer, do what needed to be done and reopen it in Photo.