I've had trouble with this. If I burn a movie and the subtitles together as data files to a CD or DVD, everything is fine.
But if I just burn the movie, and later try to add the subtitles in another session, the disc becomes unreadable.
However, if I put a small movie clip or short, say 50MB, along with the subs so that the total amount of data to be written on the second session is something reasonable (not the ridiculously tiny .srt file), the process works.
Finally, a good use for some of the shorts posted here. 
(kidding)Does anyone have more information?