I'd suggest that you modify your plans to "permenantly add subtitles to a film whilst converting it to dvd". No point using two steps. You need to look into AVISynth. Basically create a script resizing to DVD resolution with the subs added via textsub/vobsub. Then encode that instead of your source. Really though DVD supports seperate subtitle streams, so unless you have fancy effects, you may as well author with seperate subs.
