Edit vertical videos in iMovie

Alright, so I know what you're wondering, "Why would you want to edit video shot vertically?" Well I'll explain. I run a snapchat travel vlog, I upload a lot of videos I edit in iMovie, however, when I upload videos I created in iMovie, the aspect ratio is changed and so it uploads onto snapchat as a small square in the middle and the remaining space is just black. Kind of like if you watched horizontally shot footage on a vertical iPhone screen. It puts the footage in the middle and fills in the void with black emptiness. I want to keep the video the same size of the vertical screen so I can upload to Snapchat and have my edited video fill the screen. Is there any way to do this? If I try to adjust the aspect ratio in HandBrake after editing, it makes the video all skawompy and squeezed together. Thanks for any advice.