you know. I've thought up a few ways, but inevitably, I end up asking "Why?" The router has ethernet interfaces...the modem has an ethernet interface. so really, the only scenario where this would even be necessary is if the PCs had no ethernet cards, in which case, the router has just become a paperweight.
In that case, I'd get a USB hub plug all the PCS into it, and then wire modem to free usb port on a PC that will serve as a master. Then, cross my fingers and try to configure USB-USB bridge mode and network out that way. Then I'd throw the router at the interviewer's head for being stupid.
In truth, the best solution is to try and sell some ethernet cards to the customer. If it's OMGMUSTHAVEUSB, I'd sell out some USB->802.11 adapters , line out an AP to the router and just wifi it.
Or you could grab out a spec sheet and try and solder together a usb->ethernet converter. because, like I said, you always end up asking yourself....wtf?
I would call up and ask for the answer cuz now I wanna know wtf she was thinking.