You've got 2 choices for extending the UI in Explorer - you can either create a DockWindow Add-in or you can create Button Add-in from which you can launch a windows form. In both cases you can add WPF controls to the canvas should you wish to, but they must be contained within ElementHost controls.
One of my colleagues developed an Overview map DockWindow Add-in which demonstrates the use of the WPF MapControl from the ArcGIS for Silverlight/WPF API. He found that not all assembly references were resolved automatically, so had to add some extra code to do this.
