There do exist USB-to-optical converters that should work in a setup like this, though a product like this isn't very widespread. We don't really have any recommendations on a specific brand or model to use, but a quick Google/Amazon search should yield results.

Another solution is to use an external DAC, like our D1 24-bit DAC, connected to the USB port on your computer and plug it into either of the analog inputs of your HD6s.