You need something like the HagUSB from Jim Hagerman or Blue Circle's "Thingy" (their name, not mine). I believe there is a thingy on Agon for sale now.
Direct USB computer to coaxial outboard DAC
I am looking for a way to bypass a sonos or apple TV. I want to know if there is a cable that goes from computer usb to either a coaxial or toslink connection in outboard DAC? If not possible what would be a cheap alternative without sacrificing sound quality?