Adding SPDIF or USB out to a digital piano

[timlein]

I have a digital piano (a Kawai MP9000) with typical analog audio outputs. No digital audio outs (spdif or USB). I would like to modify the unit to have digital audio out so I can hook the unit up to an external DAC. Either spidif or USB outs (whichever easier). Is there a circuit I can make and attach to the digital lines in the unit to accomplish this? Enclosed is a schematic of the internal DAC circuit in the piano. I can also provide the data sheet for the DAC (MN35502) if needed.