According to Heard, he tried licensing, buying, anything he could:
http://bruce-heard.blogspot.com/2013/04/OSK.html?m=1
Again, it doesn't mean you won't be able to. But the whole reason he came up with and wrote Calidar was because they wouldn't let him work with Mystara.

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.