Originally Posted by
StargazeCyclist
Would it be feasible to take a 8 speed HG-50 and add a cog somewhere?
No, but you can mix and match some 9 speed cassettes:
https://www.sheldonbrown.com/k9.shtml#custom
For example, take 11-13-15-17-19-21-24-28-32, remove 11 and 13, add a top position 13 (or 14) and 36. This will make a great touring cassette, 13(14)-15-17-19-21-24-28-32-36.