Hmmm... So you want the Menu button to be seperated from the cancel button? Why would you want to do that?
Install Tons of Add-ons by Blizzard. Then go to your Scene_Map script and press CTRL+F. Now look for:
if Input.trigger?(Input::B)
and replace it with
if Input.trigger?(Input::Key['YOUR_KEY_HERE'])
This should work. However, I don't know if message windows are affected by pressing B. If that's the case, you would need something a little bit more complicated (not complicated at all, just a little bit more
).
I haven't tested this/looked at the Scene_Map script in a while, so I hope it works
.