Does not appear to be easily done. Workaround:
1. Load a black picture on the phone created on your Mac, or take a picture of a black surface.
2. Add the black picture at the end of the video and you will automatically get a fade to black as iMovie will add the fade transition automatically.
This also has the benefit of allowing you to add credits to a black screen if you first create a clip from the black picture and save that to your Camera Roll and then you can keep it on the phone to use in all your videos, so only need to add workaround once.