You can customize the Shutdown command already if you use the "Classic" style or "Classic with two columns" style. Go to Customize Start Menu tab and locate Shutdown in the left column (you can even edit "Shutdown dialog itself). Double click and an Edit Menu Item dialog will come up. You can specify your own command for Shutdown there.
The Windows 7 style menu does not provide a way to specify custom Shutdown command because it is designed to be easy to configure.
If all you want is Numlock to be turned on or off after turning on after Shutdown, see this article:
http://winaero.com/blog/enable-numlock- ... indows-10/