I've seen a vintage 20 button turned into a 21 by Dipper, where the added button (right side) was C# both directions. (At least, I'm pretty sure it was a Dipper modification...)
I thrice added a left-thumb button to an already-built Morse, which is a "relatively easy" thing to do -- twice for my own two Morse Ceilis and once for a customer. This took a lot of time the first time I did it, on my own for one of mine, in which I was figuring it out as I went along. Once I'd done that, subsequent operations were simpler but probably still took 4-6 hours? This is relatively easy in that the Morse design is such that potential for a left-thumb button is planned for in the design -- there's space left in the reedpan for the reed, space left in the action for the button, and the fretwork has the buttonhole already. Still, it's a fair bit of surgery to do the job.
I once added a right-hand button to a Morse anglo, as a personal project. It was a huge job. Probably took 30 hours, including the planning, making a breadboard mockup, designing some jigs, and particularly doing CAD drawing to carefully cut out a section of fretwork, create a new piece that exactly fits in, and then finishing that so it (mostly) matches and blends in with the rest of the instrument. Adrian's right, the cost would likely equal or exceed the difference in cost between a 20 and 26/30 button instrument.
Edited by wayman, 17 May 2017 - 09:19 AM.