Great question. Yes, you can add additional video frames to your editing timeline in many
creative ways.

Here’s a few I personally like to use:

1. Add a high quality jpeg image to best illustrate what you are trying to convey on-screen.
   This intensifies the point you’re trying to get across in the voice over as well.
2. If you’re working with ONLY video footage- I would recommend creating a “Freeze Frame” of the video content.
   It has the same effect as adding an image, but it’s an image of the video content instead.

   To do this in iMovie, check out this tutorial from the Apple Support Page:
   https://support.apple.com/kb/PH22927?viewlocale=en_US&locale=en_US

3. Try slowing down or speeding up your footage using “time remapping”.

   This allows you to extend the footage or shorten it as needed for the length of your voice over.

   Here’s a link that explains how to “time remap” in iMovie:
   https://support.apple.com/kb/PH22933?viewlocale=en_CA&locale=en_CA

Hope this helps you out!
Please let us know if you have any other questions.