Certainly, you may use any custom datatypes that must pass between your model and view tiers. VOs and Enums are just handy patterns to use if you are starting from scratch.
In the case you mention, XIFF, you have custom datatypes like Roster, Message and Presence. These typically will have to make it across the tiers to a view.
Fortunately, Dave Keen has put together a very keen (heh.. heh..) PureMVC/XIFF demo which you can check out here:
http://www.actionscriptdeveloper.co.uk/puremvc-tutorial-flex-puremvc-jabber-and-xiff-3-introduction/-=Cliff>