Adding custom user data within the roster

I’ve created my own top-level chat client using the SparkWebCore and XIFF components…

The next thing I want to do is to add some extra data to the roster. In this case it’s the url of an animation that that user chooses, and other users see when they chat with the user.

I’ve found the RosterExtension class, and see how the Roster class uses it… but I’m not sure how to add my own small extension to the roster.

How can I add my own fields to the Roster?