Sure. Use LPCM output just as the PS3 does when playing Secondary Audio.
To mix Secondary Audio, the primary audio track has to be decoded (just as is the case for LPCM output), then the Secondary Audio gets mixed in. Then if you've set Bitstream output, the mixed result has to be RE-encoded back into a Bitstream for output. No consumer device has the horsepower to encode a lossless track on the fly. In the case of the OPPO 95, it uses "full bit rate" (1536Kbps) DTS 5.1 for the re-encode.
If you use LPCM output, no such re-encode is needed and so you get the full quality of the decoded primary track -- with the Secondary audio mixed in when present.