A:
The Model that has the capability for the extra battery pack is the "APC Power Saving Back-UPS Pro 1500" (BR1500G) and the battery pack is "APC Back-UPS Pro External Battery Pack (for 1500VA Back-UPS Pro models)"BR24BPG. (
The unit will provide up to 865 W of power.
I don't believe there is any way connect both to usb for auto shutdown, but you would have to contact APC for a definite answer.
http://www.apc.com/shop/us/en/products/APC-Back-UPS-Pro-External-Battery-Pack-for-1500VA-Back-UPS-Pro-models-/P-BR24BPG
http://www.apc.com/shop/us/en/products/APC-Power-Saving-Back-UPS-Pro-1500/P-BR1500G