Assign shortcut for "Look Up" in context menu
You could try adding the keyboard shortcut for "Look Up in Dictionary" 



Once you have that set, you can add a dictation command (assuming you have advanced dictation enabled) by just selecting any text and speak the phrase "make this speakable" which will open this dialog box..



Use what ever phrase you want, then select keyboard shortcut in the drop-down menu and enter in the same shortcut you set in your system preferences.  Now you have a keyboard shortcut and a dictation command