How to add a TreeViewItem in the middle of two HierarchicalDataTemplates?

Question

I have a Tree View that looks like this:

The data from this tree view is coming from a ViewModel with nested objects. The classes of this objects looks like this. I changed some of the code for privacy purposes:

public class RegistroModel {
    public ObservableCollection < ClassA > ClassA { get; set; };
}

public class ClassA {
    public ObservableCollection < ClassB > ClassB { get; set; }
}

public class ClassB {
    public ClassC ClassC { get; set; }
}

The xaml of this TreeView looks like this:

   <TreeView ItemsSource="{Binding Registros}" TreeViewItem.Selected="TreeView_Selected" >
    <TreeView.Resources>
        <HierarchicalDataTemplate DataType="{x:Type vm:RegistroModel}" ItemsSource="{Binding ClassA}">
            <TextBlock Grid.Column="1" Text="{Binding Name}"/>
        </HierarchicalDataTemplate>
        <HierarchicalDataTemplate DataType="{x:Type entities:ClassA}" ItemsSource="{Binding ClassB}">
            <TextBlock Text="{Binding Name}"/>
        </HierarchicalDataTemplate>
        <HierarchicalDataTemplate DataType="{x:Type entities:ClassB}">
            <TextBlock Text="{Binding ClassC.Name}"/>
        </HierarchicalDataTemplate>
    </TreeView.Resources>
</TreeView>

Now, what I want to do, is to wrap some HierarchichalDataTemplates within a expandable Label to look like this:

I want to wrap the HierarchicalDataTemplate of RegistroModel with the label INGRESOS, the one corresponding to the ClassA model with CAUSAS PENALES label and the one with ClassB with DELITOS label. I don't know if I have to use Labels o TreeViewItems to wrap the HierarchicalDataTemplate.

Is there a way to do this without changing the ViewModel structure and only changing the XAML? What is the best and easiest way to do this?