Everything I've read says that you can't and furthermore, that there are no plans to make Cufon justifiable.
https://github.com/sorccu/cufon/wiki/Known-bugs-and-issues
A bummer, 'cause I think it's the most elegant solution solution to typeface replacement that I've seen--great text rendering regardless of the browser's rendering engine (or lack-there-of), and easy to implement. So, I'm using sIFR. I'd really love to be using Cufon, though... Before I walk away from it--I want to be sure there's no way to justify the text... Any thoughts?