Upload & Sell: On
i hadn't checked the specs, but i have hooked up a CP-E4 to my 600EX-RT and it worked fine.
From Canon's product page (Specs) for the 600EX-RT (http://www.usa.canon.com/cusa/consumer/products/cameras/speedlite_flash_lineup/speedlite_600ex_rt#Specifications)
External Power Source
Compact Battery Pack CP-E4
well, OK, i just NOW checked the specs