In reply to Martin W:
Depends on what editor you will be using. If you have something basic it may not let you layer video tracks, which is ideally what you want to do. One option might be to have the portrait video playing in 1/3 of the landscape screen and at the same time in the other 2/3, place selected still images.
It's amazing the number of folk who video things in portrait format, so I wouldn't be too hard on them. It's probably because it's easier to hold a smart phone that way, but who knows, you'd think they'd still realise what they were doing!