It is very possible to add an outboard DAC to the Sonos. You have two choices. One is a Toslink cable and the other is an SPDIF cable. My personal preference is the Toslink since it eliminates any form of electrical interference induced into the cable and thereby makes cable routing easier.
The Sonos is merely passing the bitstream from the source. It is not processing unless you use the built-in DAC, otherwise it just passes it through without any kind of processing. Tests have been done and it has been proved that it will pass bit for bit copies just fine. Of course there are people who will argue that, but for all intent and purpose, it works without any audible degradation.
I've been using the Grant Fidelity TubeDAC 0.9, TubeDAC 1.1 and Maverick Audio's TubeMagic D1. They also are excellent as preamp stages so I usually let them do the volume duties and just leave the pot on the Stereomour at 100%.
I have four Sonos boxes here. Two are hardwired into the existing ethernet and two are wireless. Can't tell them apart. At times, all are playing the same thing. All have DACs. I used one without a DAC and plugged it into my Transcendent Sound Grounded Grid Preamp hooked up to the Bottlehead Paramount 300B monoblocks and was appalled. It sounded awful in comparison to the other rigs. As soon as a DAC went back into the circuit it was marvelous! Be prepared in case you decide to make a comparison test. It's a no-brainer!