OK, now I totally know what you're saying. xD
Yes, you're right. You can create some custom HMs in this way if you know some ASM and script. However, as there're only two empty slots after "HM08", it will be very annoying to create more HMs Because the game use some complex way to do the system so we should fix enormous functions and deal with some limiters and tables. That's very boring so I simply don't do it.
Therefore, I only add the ROCK CLIMB to the game and I don't think we need more HMs xD
But as I've said, if you know ASM and script, you can succeed in doing it absolutely because that's "boring" instead of "impossible".