I don't know how often I repeated this answer on this board already.

Everything is possible.

It's just not easy.

A better question would probably be 'How feasible is it to fix the controls through rom hacking?'
As for that answer, it varies widely from game to game, like almost everything else in rom hacking. In general though, I think the hardest part would be figuring out how the controls work to begin with and planning changes correctly. I've really had no significant problems at all changing or adding new controls for Super Metroid - generally my problem is making the controls easy and relatively intuitive without giving other stuff up when I only have ~6 buttons to work with. On the other hand, I also have the game extremely well documented, so it's more like working with source code - I usually know where I want to place button checks and what I want the checks to do.

Obviously though, if you don't know assembly very well, you're not going to get anywhere. But overall there's not *too* much work with changing controls - given a game I know nothing about I could probably manage a few changes in controls within a solid day of work.
I'm not volunteering though - I have enough on my plate already.

It's definately possible to fix controls with ROMhacking. I had a friend who made a game genie code for me for Robocop 2 for NES. He fixed it so that the character don't keep sliding when moving. Cause the control is bad in that game and you sometimes suffer unfair deaths by sliding into pits that you should of been able to avoid.