I have this adapter cable thingy, I think it actually came with a really low-budget PSU, that basically takes the 8-pin CPU +12V EPS12V cable from PSU, splits off a 6-pin PCI-E power connector, and passes through to another 8-pin CPU +12V EPS12V connector to plug into mobo.
I suppose, with one of thosee connectors, you could then convert the 6-pin PCI-E to something else. But that wouldn't work in your case, because the USB card needs +5V, and that cable only splits off the +12V.