Uhh..yes it can. Scanners can be set to add a white border. On some scanners that is the default setting.
What you see in Obama's scan is the full document
You have absolutely no way of knowing that.
Feel free to document this with a link. The border around the alleged COLB is not the same width all the way around and not the same color. A digitally added border would be. Sorry, but this claim fails. Plus, you can see the shadow of the edge of the document in the scan. The document in the factlack photos doesn't match.