A behavior can be added to the behaviors collection coming from System.Windows.Interactivity.Interaction.GetBehaviors.
Example:
System.Windows.Interactivity.Behavior behavior = new ESRI.ArcGIS.Client.Behaviors.ConstrainExtentBehavior()
{
ConstrainedExtent = new ESRI.ArcGIS.Client.Geometry.Envelope(-120, 30, -60, 60)
};
System.Windows.Interactivity.Interaction.GetBehaviors(MyMap).Add(behavior);