Adding New Items on Factory Toolbars
The screenshot below shows the two factory toolbars: Toolbar1 and Toolbar2

You can add new buttons to the factory toolbars either in XAML or programmatically (C#, VB.NET).
Adding new buttons in XAML
Example of two additional buttons in the toolbars is below:
<wpfHtmlEditor:WpfHtmlEditor x:Name="WpfHtmlEditor" Grid.Row="1">
<wpfHtmlEditor:WpfHtmlEditor.Toolbar1Items>
<Button Style="{StaticResource {x:Static ToolBar.ButtonStyleKey}}" Click="AdditionalButton1_OnClick">Additional button1</Button>
</wpfHtmlEditor:WpfHtmlEditor.Toolbar1Items>
<wpfHtmlEditor:WpfHtmlEditor.Toolbar2Items>
<Button Style="{StaticResource {x:Static ToolBar.ButtonStyleKey}}" Click="AdditionalButton2_OnClick">Additional button2</Button>
</wpfHtmlEditor:WpfHtmlEditor.Toolbar2Items>
</wpfHtmlEditor:WpfHtmlEditor>Collections Toolbar1Items and Toolbar2Items allow you to add additional buttons on Toolbar1 and Toolbar2 accordingly. Result is below:

Adding new buttons programmatically
You can add buttons programmatically to the toolbars already available in the HTML Editor. The WpfHtmlEditor class provides two properties – Toolbar1 and Toolbar2.
In the code snippet below, we have added a Signature button to the first Toolbar (Toolbar1). This button inserts a predefined HTML text in the Editor at the current cursor location.
C#
private void Window1_OnLoaded(object sender, RoutedEventArgs e)
{
Button btnSignature = new Button();
btnSignature.Content = "Signature";
btnSignature.Click += btnSignature_Click;
btnSignature.BorderBrush = Brushes.Black;
btnSignature.BorderThickness = new Thickness(1);
wpfHtmlEditor1.Toolbar1.ToolBar.Items.Add(btnSignature);
}
void btnSignature_Click(object sender, RoutedEventArgs e)
{
wpfHtmlEditor1.Content.InsertHtml("<p>James Paul <br/><span style=\"color:blue;\">Executive Officer</span><br />james_paul@some_domain.com</p>", keepSelected: false);
}VB.NET
Private Sub Window1_Loaded(sender As Object, e As RoutedEventArgs)
Dim btnSignature As New Button()
btnSignature.Content = "Signature"
AddHandler btnSignature.Click, AddressOf btnSignature_Click
btnSignature.BorderBrush = Brushes.Black
btnSignature.BorderThickness = New Thickness(1)
htmlEditor1.Toolbar1.Toolbar.Items.Add(btnSignature)
End Sub
Private Sub btnSignature_Click(sender As Object, e As RoutedEventArgs)
htmlEditor1.Content.InsertHtml("<p>James Paul <br/><span style=""color:blue;"">Executive Officer</span><br />james_paul@some_domain.com</p>", False)
End Sub