Mixing digital and analog input to digital output

I'm looking for a way to mix a single I2S signal with a single stereo analog input, and output the mixed result either as I2S or S/PDIF (S/PDIF would be preferred, as that is my actual target output format, but if I have to I can convert I2S->S/PDIF easily enough.  I've found plenty of solutions for mixing digital and analog inputs with analog output, but are there any options for digital output?