I am not an expert on iMovie (I use different software) but I did do a small project to see if I could do what you want.
What I did was create the trailer first then when you have it the way you want share it out as a file. It will create it in the Movies folder. Now import it into a new project, select all of it and drag it down into the timeline. Now add your other movie footage after the trailer. You will probably want to put a transition between the two. When you are finished share the entire project out how ever you want.
Also here is a site with some free video tutorials on how to use iMovie: