Expression doesn't make this easy, but you can hand-edit the XAML to do it.
(If this has answered your question, "Mark as Answer")
Shawn Wildermuth
C# MVP, MCSD, Speaker and Author
Silverlight 2 Workshop
September 15-17, 2008 - Denver, CO
September 22-24, 2008 - New York, NY
http://www.silverlight-tour.com