You could only do it by adding white (or another plain color) above and below the photo. Have a look at the examples on page 155 (latest PDF of Edit Like a Pro) or page 231 in the Missing FAQ) to illustrate it.
Well sometimes you need to crop a picture but do not want to eliminate any important item. So in that case what I would do is that I would crop in such a ratio where just the unimportant part gets removed. Sometimes I consider my hair getting cropped just to keep the size.