you are refering to usb pen drives, right?
these devices don't need a lot of power.

I still don't have a 770 so I don't know if this aplies, but i find it very strange that the usb connector doesn't have a +5v connector...
(usb uses 4 pins: +5v; usb_N; usb_P; gnd.)

anyway, the power connector (of the 770) is just next to it...
It "may" be possible to connect a plug to this power connector, and "leech" power from it. (i don't know for sure).
If it has 4.5 or 5v, then there's no need for a conversion... but if it's 3.3v, we need a small circuit to generate 5v from it.