If the emulator itself does not support it and you don't want to recode the emulator to handle it (it is probably not a trivial mod, though if your did it then you might be able to get it into the code rather than having to compile a new one each time) then you might look into keyboard macros. https://autohotkey.com/
looks like a good start. Most are designed to have one button input multiple buttons/sequences and such but there is nothing stopping you from having multiple macros that are just one button to ultimately press the same "button". It should be fast enough for use on games (many use it for such things) but be aware it might be an issue.
Alternatively a lot of things with only one input might still have the option to input a joypad press instead. If you can figure out a way to fake a joypad button with a keyboard (most usually go the other way and have the joypad fake a keyboard and mouse so I have no suggestions here) you might get somewhere.