I believe that the program "alien" will take care of that for you. It allows conversion between .rpm, .deb, .tgz and a few other formats. I don't know where you'd find the program itself, but maybe a google search will work well enough.
This site has the source for alien, but maybe you don't want to compile it. Just search around on google linux or maybe its already installed or you can find it easily with some cd that you have, etc. Good luck.