I think you would use a reviver or AC12 as a converter. Why not just use SPDIF 2 input if 218 to non speakerlink DSPs? 861 SL out to 218 SL in routes 861 PCM to 218 for unfold up to 96k/88k then to DSPs. To me in parallel would have a separate route from 861 directly to the other DSP input.

James
NC1: 861v8, ID41, 218, 251(to outdoors), DSP8000SEUG, 7200SEC, DSP520 rears, Apple TV, DirecTV, Oppo BD103, QNAP251, Roon
NC2-M7005, Triad Silver Speakers,Acurus Amps

FL1: 861v4, ID40, 218, MS200, DSP8000SEs, DSP7200SEC, DSP5000s, DirecTv, AppleTv, OppoBD103, QNAP469, Roon
FL2: Marantz 7005, 251, M60s,60C,33s, Oppo BDP-93,MS200, Roon