This isn't really an expansion per say. It takes the numerous backstories of the wizard, which is from several books, and turns it into a singular movie.
It cherry picks though, as in one book the wizard is a dick (he came to power by killing Ozma's parents) and other, later novels, where he's a goofy nice guy who came to power because Ozma's parents were killed. Baum as an author was anything but consistent. Hell, Central City (The Emerald city most call it), isn't even Emerald in some books, it just has green rooftops and brick buildings.
If you enjoy Oz as a world though, I highly suggest reading Marvel's line of graphic novels. They're great.