Not quite sure what you are trying to do.
If you have a vertical portrait with black sides you can place anything on the track below the portrait track and it will take the place of the black sides.
Or you can click the pan/crop symbol in the portrait event, then right click the work field in the pan crop window which will bring up a menu, then choose match output aspect and the black sides will disappear but you will alos lose some of the vertical aspect of the picture. You can move it around to have the important part visible.
If you use the masking tool in pan/crop you can make any shape mask you want.