Примерно так:
var style = new Style(typeof(Ellipse));
var trigger = new DataTrigger();
trigger.Binding = new Binding("Opacity") { ElementName = "ellipse1" };
trigger.Value = 0.5;
Storyboard sb = new Storyboard();
//Add animation to sb, note the attached storyboard properties which are set with static methods:
//Storyboard.SetTarget(...);
//Storyboard.SetTargetProperty(...);
//Storyboard.SetTargetName(...);
trigger.EnterActions.Add(new BeginStoryboard() { Storyboard = sb });
style.Triggers.Add(trigger);