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>