ok, thanks for clarifying. I dont think thats possible. As far as i have seen, app inventor has a built in function for the menu button. when pressed it opens the appinventor menu that offers you to exit the app or see the about screen.
the only buttons you can manage or edit are the buttons you create in app inventor or the physical (on phone) back button. in this case, for the back button, you will tell the app to do something when the back button on the phone is pressed and it will do the action, like you can say "when back button pressed, play a sound or go to a different screen or download a file".
you can check it out in the blocks screen