If your powerpack only has one 8pin connector you can get a molex like the one in the picture below, also just make sure the power pack can deliver the exact watts needed for the GPU and motherboard to run perfectly
Yes I know I can do that but I don't think it's necessary. I think the extra 8 pin was just in case kind of thing to cover themselves legally. Every connector is a point of failure so I prefer to minimise them as much as possible. The question remains, is the card hardwired not to work without power supplied to both connectors?