I finally finished my video!

I'm so pleased with it, and I have just one more question. Four attempts to contact him later, the coordinator of the AMV contest I'm entering finally wrote me back. He recommends adding letterboxing to my footage, since it is widescreen (Miyazki's Spirited Away, to be exact). My original plan was to simply save the video in 4:3 ratio (my program then automatically adds letterboxing) That was fine until I got to the parts where I used a still frame. In those spots, even though in the preview screen it looked fine, in the final product, the images were stretched vertically to look full screen. This was remedied by rendering the project with a 16:9 aspect ratio, but this again leaves me without the letterboxing.
Of course, I could always use my video as source footage, stick the entire thing in the editor again, and save it in 4:3, but this, of course lowers the quality some. I normally wouldn't have as much of a problem with a slight quality loss, but this is for a convention, so I'm being anal.
Is there a way to add letterboxing to an already rendered video (It's in DV format, if it helps.)
For the record, the contest is accepting videos in:
MPEG-1, MPEG-2, Huffy, DV, Xvid or Uncompressed
if that information is of any help.
Thanks so much for the help.
"They say that there is no medicine that can cure a fool... I guess that's true." (Potion shop's old woman- LoZ OoT)