Is there way how to backup original DVD and add your own subtitles to it? If so can someone tell me how to do it ?
It believe it is possible, but it requires you to reauthor parts of the DVD structure- no small task.
As you can see from the comparison chart at the bottom, some of those programs support adding up to 32 subtitle tracks. Although that guide is for authoring original content, you should be able to rip an existing DVD to its constituent parts and open it in one of those programs for editing.
Please see my guide from http://forum.doom9.org/showthread.php?threadid=75666
Or the new and better guide from http://rodivx.xro.us
I have released a new guide on How to add subtitles to a DVD (using DVD-lab PRO). You can find it here ! I made also a MOVIE Tutorial, showing to people how to insert subtitles to a DVD.
P.S.: THE OLD LINKS ARE NOT WORKING ANYMORE !