How to create custom Keyboard Shortcuts?
Assuming you're on a normal Mac then the power button will bring it up. Details here.

Further details about creating shortcuts here.

You can use the info in that document to create a short-cut in any app for whatever key combo you want for the Shutdown... item.

Simplified procedure:


Open System Preferences -> Keyboard -> Shortcuts tab
From the left, click App Shortcuts.
Click +
Choose 'All Applications' as the app
Choose 'Shut Down...' as the command
Put in whatever shortcut you want.