This is an example:

        Dim ctrl As System.Web.UI.WebControls.WebControl
        ctrl = Editor1.CreateCommandButton("MyButton", "text.gif", "Insert My Custom Text")

        ctrl.Attributes("onclick") = "ShowMyDialog(this)"

        'add this custom button into the editor
        Editor1.InsertToolControl(pos, "MyButton", ctrl)

 

Demo: http://cutesoft.net/example/howto/AddButtons/vb/create_a_custom_button.aspx

 

You can find the source code of the above example in the download package.