the n800 doesn't have very much power available on its usb port, barely enough to drive a lower-power flash drive (it won't even drive my microSD card in a Sandisk USB card reader), so I doubt you'd get enough to run a usb wifi adaptor.

my guess is that if you're really dedicated, adding an external socket is viable. Someone added an external wifi antenna to their Zaurus 6000. Mind you, it was a pretty fugly hack!