private void AddSplitButton(RibbonPanel panel)
{
    string assembly = @"D:\Sample\HelloWorld\bin\Debug\HelloWorld.dll";

    // create push buttons for split button drop down
    PushButtonData bOne = new PushButtonData("ButtonNameA", "Option One",
        assembly, "Hello.HelloOne");
    bOne.LargeImage = 
        new BitmapImage(new Uri(@"D:\Sample\HelloWorld\bin\Debug\One.bmp"));

    PushButtonData bTwo = new PushButtonData("ButtonNameB", "Option Two",
        assembly, "Hello.HelloTwo");
    bTwo.LargeImage = 
        new BitmapImage(new Uri(@"D:\Sample\HelloWorld\bin\Debug\Two.bmp"));

    PushButtonData bThree = new PushButtonData("ButtonNameC", "Option Three",
        assembly, "Hello.HelloThree");
    bThree.LargeImage = 
        new BitmapImage(new Uri(@"D:\Sample\HelloWorld\bin\Debug\Three.bmp"));

    SplitButtonData sb1 = new SplitButtonData("splitButton1", "Split");
    SplitButton sb = panel.AddItem(sb1) as SplitButton;
    sb.AddPushButton(bOne);
    sb.AddPushButton(bTwo);
    sb.AddPushButton(bThree);
}

Private Sub AddSplitButton(panel As RibbonPanel)
   Dim assembly As String = "D:\Sample\HelloWorld\bin\Debug\HelloWorld.dll"

   ' create push buttons for split button drop down
   Dim bOne As New PushButtonData("ButtonNameA", "Option One", assembly, "Hello.HelloOne")
   bOne.LargeImage = New BitmapImage(New Uri("D:\Sample\HelloWorld\bin\Debug\One.bmp"))

   Dim bTwo As New PushButtonData("ButtonNameB", "Option Two", assembly, "Hello.HelloTwo")
   bTwo.LargeImage = New BitmapImage(New Uri("D:\Sample\HelloWorld\bin\Debug\Two.bmp"))

   Dim bThree As New PushButtonData("ButtonNameC", "Option Three", assembly, "Hello.HelloThree")
   bThree.LargeImage = New BitmapImage(New Uri("D:\Sample\HelloWorld\bin\Debug\Three.bmp"))

   Dim sb1 As New SplitButtonData("splitButton1", "Split")
   Dim sb As SplitButton = TryCast(panel.AddItem(sb1), SplitButton)
   sb.AddPushButton(bOne)
   sb.AddPushButton(bTwo)
   sb.AddPushButton(bThree)
End Sub