You can add a shortcut button to any group by first selecting the group and then clicking the right mouse button in a blank part of the Shortcut bar and selecting Add To Bar from the context menu. The Select Shortcut dialog box is displayed.
From the drop down list, choose the mode containing the shortcut button that you wish to add to the selected group, select the relevant shortcut button from the Shortcuts list and press the Add button. The shortcut button will be added to the bottom of the selected group.