i'm not sure, this is the easiest way, but what i did was the other way round (converting ntsc-dvd (recorded in a dvd-video-cam) to pal (for using in an imovie project)):
convert it with qt pro!
in your case, you have to demux the mpeg file to get a mpv and a mpa file, with qt pro you combine both files, export them as .dv in ntsc and finally mux'em back to mpeg1......
i'm very sure, quality is ... a mess? the ntsc>>pal conversion with qt is ... cheap ;-)
but, remember, those ol' days (10 years ago), video standard converter costed thousands...