Convert a trailer to a movie in iMovie on Mac
You can convert a trailer to a standard movie project, which allows you to edit it in the timeline.
Note: After you convert a trailer to a movie, you can’t convert it back to a trailer. If you want to keep a copy of your trailer, duplicate it before you convert it.
Note: If you want to preserve the original pacing of the trailer, replace existing clips with video clips of the same length. To learn how to replace video, see “Replace a clip in the timeline” in Add clips to your movie in iMovie on Mac.