Change canvas size in Preview.app without scaling image?
This works well for me: 


Select all (cmd+A) and cut (cmd+X) the image from its canvas (convert to png if asked). 
Resize the image as desired from the "Tools" menu (unlock the proportions if desired)
Re-paste original image from the clipboard (cmd+V) and move it where you want it on the resized canvas
Use cmd+- if necessary to zoom out (you'll see checkerboard where the canvas is and white beyond)
Paste in other images, etc, as you like