I remembered that I had cut a sleeve off of another white shirt to make it into a short-sleeved shirt. I never finished it but I thought that I could use the sleeve material for the extension piece. Unfortunately the sleeves are different widths so I had to cut the suit shirt just above the sleeve placket to get the same width. Then came the problem of how to attach the lower edge to the new piece because they were different widths too. I decided to try the upper edge first to see if it worked before figuring out the lower one. I put the cuff piece inside the sleeve so I could sew the wrong sides together as I wanted to make a flat-felled seam. Then I pulled the cuff piece out so I could iron the new seam to have it easier to trim one of the edges before turning the other one over it.
Next is trying to narrow the shirt. Sigh.