This blog describes how to able to place controls in the header content template in threeview. see the below example you will get idea
<TreeView HorizontalAlignment="Left" Height="307" VerticalAlignment="Top" Width="397" Name="filterableTreeview">
<HeaderedContentControl> <StackPanel Width="273" Orientation="Horizontal"> <Label Content="Search"></Label> <TextBox Width="227"></TextBox> </StackPanel> </HeaderedContentControl> </TreeView> |