Exporting to (portrait) orientation is not recommended because the TVs and monitors we watch are landscape oriented. For that reason, video streaming services like Vimeo and YouTube display portrait oriented videos by squeezing them into a landscape orientation with black space on each side. This makes your video smaller.
Now, I understand that there might be some reason, like you were afflicted with VVS (Vertical Video Syndrome) one day, and you shot something very special in portrait mode. Now you’re stuck with it, and perhaps you want to add titles, or what have you.
So if you have to do it, Final Cut Pro X let’s you do it easily. When you set up your HD project, select custom from the drop down and reverse the width and height pixels. Any quality video editor should have custom property settings.
Then place your footage on the timeline and rotate 90 degrees in either way that coordinates to how you shot the footage.