You can combined geometries using a GeometryGroup. A GeometryGroup
creates a composite geometry from one or more Geometry
objects.
GeometryGroup gg = new GeometryGroup();
gg.Children.Add(new RectangleGeometry(new Rect(0, 0, 100, 100)));
gg.Children.Add(new RectangleGeometry(new Rect(100, 100, 100, 100)));
gg.Children.Add(new RectangleGeometry(new Rect(200, 200, 100, 100)));
System.Windows.Shapes.Path path = new System.Windows.Shapes.Path();
path.Data = gg;
path.Fill = Brushes.Blue;