I've not seen a USB adapter that supports WoL but they probably do exist, i know the motherboard i recently bought for my server provides power to USB ports when the system is shut down which would imply that a USB device could remain listening to the airwaves while the system is off.
In theory it's possible, whether there is the kit on the market that supports it is another question.
If you haven't already installed the adapter i'd go for a PCI / PCMCIA wifi card as they're much more likely to support it. As is the motherboard more likely to accomodate it.