USB descriptor (LUFA)
You need to edit the HID report descriptor, but also the main code. Change this portion of the HID descriptor:

    0x05, 0x09,          /*   Usage Page (Button)                              */
    0x09, 0x02,          /*   Usage (Button 2)                                 */
    0x09, 0x01,          /*   Usage (Button 1)                                 */
    0x15, 0x00,          /*   Logical Minimum (0)                              */
    0x25, 0x01,          /*   Logical Maximum (1)                              */
    0x75, 0x01,          /*   Report Size (1)                                  */
    0x95, 0x02,          /*   Report Count (2)                                 */
    0x81, 0x02,          /*   Input (Data, Variable, Absolute)                 */
    0x75, 0x06,          /*   Report Size (6)                                  */
    0x95, 0x01,          /*   Report Count (1)                                 */
    0x81, 0x01,          /*   Input (Constant)                                 */
    0xc0                 /* End Collection                                     */


To this:

    0x05, 0x09,          /*   Usage Page (Button)                              */
    0x09, 0x03,          /*   Usage (Button 3)                                 */
    0x09, 0x02,          /*   Usage (Button 2)                                 */
    0x09, 0x01,          /*   Usage (Button 1)                                 */
    0x15, 0x00,          /*   Logical Minimum (0)                              */
    0x25, 0x01,          /*   Logical Maximum (1)                              */
    0x75, 0x01,          /*   Report Size (1)                                  */
    0x95, 0x03,          /*   Report Count (3)                                 */
    0x81, 0x02,          /*   Input (Data, Variable, Absolute)                 */
    0x75, 0x06,          /*   Report Size (5)                                  */
    0x95, 0x01,          /*   Report Count (1)                                 */
    0x81, 0x01,          /*   Input (Constant)                                 */
    0xc0                 /* End Collection                                     */


And set the third bit of the Buttons element in the element in the CALLBACK_HID_Device_CreateHIDReport() function of the main source file, i.e. to "push" the new third button, use:

 if (ButtonStatus_LCL &amp; BUTTONS_BUTTON1)
   JoystickReport-&gt;Button |= (1 &lt;&lt; 2);