Well i think there is a way...
Start out with giving your storyboard a name
<Storyboard x:Name="anima"></Storyboard>
then go into the ".cs" file
//connect it to a listener
Storyboard ST = this.FindName("anima") as Storyboard;
ST.Completed += new EventHandler(AnimaListener);
//the listener
void AnimaListener(object sender, EventArgs e)
{
Microsoft.Expression.Prototyping.Services.PlayerContext.Instance.ActiveNavigationViewModel.NavigateToScreen("nameOfTheNewScreen", true);
}
http://social.expression.microsoft.com/Forums/en/blend/thread/e2970c76-6968-40db-a9c2-fca5d56f3f9f
Where i found the navigation code
http://www.eggheadcafe.com/software/aspnet/30084553/storyboard-completed-event.aspx
Where i found how to link a storyboard to a listener