Adding Digital INPUT to a CDP for use as a DAC?

Howdy! I'm curious as to if it would possible to add a digital input to a CDP not originally having one, specifically the Ah! Njoe Tjobe 4000 (Marantz), in order to use it as an external DAC for another transport/CDP/DVDP? If anyone could give me instructions as to how, or direct me to someone who could do this I would greatly appreciate it! Thanks in advance.
Save yourself the agony and pain. Find a used MSB
-DAC III for about $250. DAC I & II's are even cheaper!
I already own a MSB Nelson Dac w/ Upsampling, and I also am part masochist, so I welcome the pain and agony of this mod, if doable, and would really appreciate directions!
Get the schematics and it can be figured out. I think it's doable but a huge waste of time with this player.