Your onboard audio is also a USB soundcard, just like your DAC would be. Just the USB-audio chip is connected to the USB bus directly on the motherboard. The USB driver is very likely the same.As to the DAC has USB, why use the sound card or onboard? because i don't know how the sound is send to the Schiit via USB? With my Toslink, now w/o buying the sound card, I just connect the Toslink cable to the back of the Schitt, and the Schiit will do the rest. I don't have struggle w/ USB software / hardware connection
So with your onboard audio you would have USB transmitter - > USB receiver with PLL'ed clock (because USB adaptive) -> SPDIF transmitter-> SPDIF receiver with another PLL'd clock (because SPDIF is always clock master) -> DAC, with direct connection it would be USB transmitter - > USB receiver with precise crystal clock (because USB async) -> DAC
Just try to hook up your DAC via USB, and most likely no extra driver will be needed.