WPF Datagrid: When Frozen Columns, add any control at the Horizontal Scrollbar's blank area

Hi,

I have a FrozenColumnCount = 3 always with HorizontalScrollBar visible. Now, I have a requirement to add controls in the blank area (left of the scroll bar) -- See the RED highlighted area in the image.

How can I add any control there ? It will be great if something of this sort can be achieved via XAML (ControlTemplates) ? I am using the standard WPF v4.0 datagrid in my application.

NB: The sample can be downloaded from here -http://blogs.msdn.com/b/vinsibal/archive/2008/10/31/wpf-datagrid-frozen-row-sample.aspx