iMovie Question - Mixing Portrait iPhone videos with 1080p Widescreen

If I have an event in iMovie and majority of my footage is in widescreen; is there a way to place something behind the portrait video so I don't have huge black bars on either side during those segments? I'd even be fine with having multiple of the same footage going simultaneously side by side I just don't want the letterboxing.

This leaves me with a problem of letterboxing still, I don't know how to get those black bars and I guess even if I could have three of the same video going side-by-side that still wouldn't work because of the letterboxing.

HAHAHAHA. That has got to be one of the most hilariously relatable videos ever. In any event, I've decided to just rotate the video and make them sideways. It's just a "clubbing" video so that will have to do.