If you are just doing this for yourself, you can put the UI into customization mode (click on the small down arrows at the extreme right side of the ribbon bar or menu bar, or use the <CTRL> + 1 shortcut).
On the Customize dialog, select New Item in the Categories list. Then left click and hold on New User Command in the Items list and drag&drop it where you want the command to appear. RMB on the new button to change the name/action/etc.
Note that you can also add Menus, Groups, Galleries and Drop-downs in the same manner.