iMovie (2013): Convert a trailer
You can convert a trailer to a movie, which lets you change it in any way you want: adding or deleting video clips, changing the style of titles and transitions, applying special effects, replacing the background music, and so on.
You can convert a trailer to a movie at any time, whether or not it’s complete. When you convert a trailer, its name doesn’t change.
Convert your trailer to a movie
Select a trailer in the browser, or double-click a trailer to open it.
Choose File > Convert Trailer to Movie.
If you converted the trailer before you filled all the placeholder wells with video clips, you see placeholder clips (grayscale images) in the timeline after the conversion. You can replace these placeholders with video clips.
Edit the movie just as you would edit any other movie.