The different bars of a StackedBar are represent by just Rectangles and they don't have a container that holds them.
In some themes the Rectangles have Stroke and in others they don't have. Since this "border" comes from the Rectangle class and you can't choose on which side you want the border to be displayed the RadChart does not support this either.
However if you want you can explore the visual tree using a tool like XAML/Silverlight Spy and see for yourself if you can workaround this.
All the best,
the Telerik team