do you mean that you want to take a video file that does not have subtitles and add in subtitles of your own? if so.... you can do it if you feel like spending hours and hours re-editing the video and typing in all of the text yourself... if you are talking about extracting existing subtitles from a dvd when you rip it to your hard drive... this is possible and a heck of a lot easier... basically you need to make sure you include the subtitle track when you rip and compress the files from the dvd.
I have a DVD (full dvd movie) that does not have subtitles in my language.. i have a srt subtitle and i want to add it to the dvd..