You can add a disc brake with a Nimbus d’brake mount and KH Spirit Cranks to nearly every unicycle with ISIS interface.
The unicycle you selected has a steel frame with stamped bearing holders. Thus, you’ll need an extra adapter. This can be made of 3 washers od as a solid adapter.
http://www.unicyclist.com/forums/showpost.php?p=1584246&postcount=1167
And this is my solution with a solid adapter
In this thread we discuss the thing with the adapter/washers in english.