I know this thread is old, but after I searched for a long time, I finally found this video which allowed me to created HD-SD movies in iTunes: http://www.youtube.com/watch?v=93bG1hUCbvI&feature=related
It worked perfectly and didn't take much time, so I recommend that anyone trying to combine HD and SD videos use this tutorial.
It worked perfectly and didn't take much time, so I recommend that anyone trying to combine HD and SD videos use this tutorial.