How to create a white border around the photo?

Hi there, 

I can't figure out how to add a white border to a photo. I'm using Lightroom v3.3 (not Lightroom Classic), and have tried using the Scale slider in Geometry, but because I've cropped and used the vertical and horizontal distortion edits, it doesn't work. Is there a way to add a white border to a photo that's already been cropped? Thank you! 

Grace