Hi,
The quickest way to do this using the keyboard is by going old school.
When you open Acrobat access the File Menu via keyboard shortcut using "ALT+F" then hit letter "R" then hit letter "M"
This is the keyboard shortcut sequence to open up the Combine Files Tools without using the mouse.
If you like learning shell scripting or batch programming you can automate this keyboard sequence as a keyboard gesture using AutoHotkey. Download it and install it, learn how to use, and get creative.
But the sequence above is what you need. Very simple.
This used to be taught in computer repair classes back in the day when a mouse was broken and only keyboard was working.
With your left-hand thumb press the ALT key and hit F with your left index finger; it will quickly open up the File Menu; release your left thumb from the ALT key while moving up your left index finger one letter above letter F which in this case is R , hit it once to open up the "Create File" submenu.
Last, hit once with your right index finger letter M which will open up the Combine Files Tools. That's it. It takes less than a second to perform this sequence.
Just remember: (ALT+F) --> R --> M