List<string> mylist = new List<string>() 
        { 
        "A",
        "B",
        "C"
        };

         TreeViewItem parent1 = new TreeViewItem();
         parent1.Header = "D";

         foreach (string q in mylist)
         {
             TreeViewItem parent = new TreeViewItem();
             TreeViewItem child = new TreeViewItem();
             TreeNode tn = new TreeNode();
             string str = q;
             tn.Text = str;
             parent.Header = tn.Text.ToString();
             child.Header = "1";
             tv.Items.Add(parent);
             parent.Items.Add(child);
         }

        tv.Items.Add(parent1);


<Window x:Class="WpfApplication7.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        Title="MainWindow" Height="350" Width="525">
    <Grid>
        <StackPanel>
            <TreeView Height="302" x:Name="tv"></TreeView>
        </StackPanel>
    </Grid>
</Window>