You can't, at least not directly.
You can use other movie editing apps to export to Photos, then use those in iMovie. I took a look around the App Store and found Extras for iMovies. It works along that idea, letting you create special credits and other stuff you can import into iMovie.
If you are ready to move on to a more versatile movie editor I hear Pinnacle Studio offers more flexibly than iMovie. I don't know personally, as my movie needs are seldom and simple. iMovie is enough.