Well, essentially to do so you will need to rip the video back off the disc. You can do this with any number of utilities out there, I prefer Handbrake. (free!) Keep in mind that most of these utilities convert the video to something more managable than DV such as Xvid/Divx/mpeg4. So the quality shifts.
In some you can specify the quality via the bitrate, but you will still have to then convert that to a DV file afterwards. And what you get may not be as good quality as what you wanted.
I suggest that if you have video on your computer already and want to back it up, that you do so as a DVD-Rom, saving the files in the format they exist in.
Hope this helps.