You may use this method to manually add a button to the action_area property of the Dialog. Of course, you may also add buttons after you have called the constructor with buttons. You may add a custom button or a stock item, depending on whether you pass a stock-ID or plain text as the string parameter. The response signal may also be indicated as given in GtkResponseTypes.
The created button is returned as a widget by this method, but mostly you wouldn't need to use it.