Really easy answer for you Mike - especially if you have a newer Mac that comes with the complete (Pro) version of QuickTime.
In QuickTime Pro, simply copying and pasting (yes- just like text) will do what you want. Saving does still take some time once a movie is appended. Just bring the movie you want to append to the foreground, hit command-c and then go back to the movie you want to append to, move the playhead indicator to the end and press command-v.
(I wonder though if you don't have other issues with your iMovie/QuickTime installation if iMovie is crapping out on you before the end? Do you have Perian or any other 3rd party codec pack installed that could be causing problems?)