I'm not sure what you mean here.

If the original movie had subtitles, they will still be there after re-authoring, unless you deliberately choose to remove them of course.

If, however, you're talking about adding subtitles to a movie that never had them in the first place, then you'll need an upmarket editor and a whole lot of time on your hands.

My reply assumes you are referring to a finished DVD in your query and not some Nero Digital DivX backyard special.

-Pete