I'm pretty sure this isn't possible, adorners are always drawn on top of the adorned element.
See the Adorners Overview on MSDN
"An Adorner is a custom FrameworkElement that is bound to a UIElement. Adorners are rendered in an AdornerLayer, which is a rendering surface that is always on top of the adorned element or a collection of adorned elements"
You could probably get the same effect by adorning with a normal border that just borders the adorned element instead of trying to stick it behind the adorned element.