Code

public class GeometryConverter : IValueConverter
{
	public object Convert(object value, Type targetType, object parameter,
		System.Globalization.CultureInfo culture)
	{
		if (targetType == typeof(Geometry))
		{
			ShapeNode src = value as ShapeNode;
			if (src != null)
				return src.Shape.CreateOutlineGeometry(src.Bounds.Width, src.Bounds.Height);
		}
		return value;
	}

	public object ConvertBack(object value, Type targetType, object parameter,
		System.Globalization.CultureInfo culture)
	{
		throw new NotImplementedException();
	}
}

<Canvas.Resources>
	<local:GeometryConverter x:Key="geometryConverter"/>
</Canvas.Resources>

<Path Canvas.Left="30" Canvas.Top="30"
	Data="{Binding Path=., RelativeSource={RelativeSource TemplatedParent}, Converter={StaticResource geometryConverter}}"
	Stroke="{TemplateBinding Stroke}"
	Fill="Black" />