Let me help you here.
This has been a question that has frequently surfaced.
BattleClinic has done a lot of research and concluded that there is no direct conversion. I've spent close to $2000 US to figure this out. I've tried a staggering variety of creative approaches, including even analog video-out to a VCR. Heh...what a mess that was. Anyway...
To convert the files, you'll need Adobe Premier and a decent screencap program like Camtasia Studio, plus an extremely high-end PC since the load required to run the game + the screencapture program will be enormous. You'll also need several specific codecs which allow directx captures, and you'll need at least a few GB if your game goes over 15 minutes.
I am able to tax my Dell XPS with a P4 3.0ghz / 1.5GB RAM / ATI RADEON 9800 Pro to the limit just to get a decent framerate on captures inside of a direct-x game.
Hope that helps.