Thanks for the response everyone. Just to clarify I want to take a screencast, prepend some pre-generated video, append pre-generated audio and dub a separately

Thanks for the response everyone. Just to clarify I want to take a screencast, prepend some pre-generated video, append pre-generated audio and dub a separately recorded audio track. Ideally I'd do this in something quick like iMovie unless anyone knows of anything better. Features that would be nice to have would be zooming in on particular areas and zooming out.