I'm not sure what exactly you want to do.
If you're talking about .srt subtitle-files and playing these with BS.Player, the only thing to do is to save the .srt file in the same folder as the movie-file and name it identical to the movie-file (except for the extension of course). You can also burn both the movie-file and .srt-file to a (data) CD/DVD. Example:
MyMovie.avi respectively
MyMovie.srt .
When playing the movie BS.Player will automatically detect and display the subtitle.
However, if your talking about embedding or "hard-burning" the sub with the movie-file; such a job isn't one of the features of BS
.Player. There are several ways to achieve that. Just google for it.
Some links:
http://forum.bsplayer.com/subtitles/...html#post41859 http://forum.bsplayer.com/answered-s...html#post40990 http://forum.bsplayer.com/general-ta...html#post34443 New Avi2DVD guide added! - AfterDawn
(Note 'New' meanwhile is rather old already. This is an old google result. I'm sure a new google-search would come up with more up-to-date results; also in French.)