You can’t do it with keyboard shortcuts as it is not a menu item.
I want to this as I have assigned the other align commands to my WACOM buttons:
I made a complex keyboard shortcut for each of the commands in Illustrator (so I don’t press it accidentally). Then using the WACOM sys pref pane I then just assigned that complex button press to the key that I want. Using different combinations of modifiers I can assign all the keys and select different things using SHIFT.
But I can’t do this with the highlighted items as they don’t appear in the Keyboard Shortcuts menu. I don’t like having to open up that little tag every time I want to change the mode.