Subtitles are not in the original disk, therefore not SBS.
3Dsubtitler is freeware downloadable from
http://www.videohelp.com/tools/3DSubtitler which converts .srt subtitles to idx or sup.
I use subtitles a lot because of some hearing loss. Some subtitles are better than others, so I occasionally replace them (or add).
Oops! My bad. 3Dsubtitler does create SBS. The question is: at what stage do you mux these in?