Thx a lot Boyd, I added .Add(xtpControlButton, 35001, "Adapt...") in the frmLoad and it works.

There's no straightforward way to do this.  My workaround is to create a dummy popup menu and then add a control to that menu that has the same ID as the 'Customize' control that appears on the built-in customize menu.  You can then use the 'Execute' method of that control to trigger the 'Customize' dialog.  I just wrapped the routine up in a function so that I can easily call it for any of my CommandBars instances.  I've been using this approach without any problems whatsoever.  Here's some sample code:

Public Function CustomizeToolbar(ByRef cb as CommandBars)
    Dim TempBar As CommandBar, Control As CommandBarControl
    If cb Is Nothing Then Exit Sub
    Set TempBar = cb.Add("Temp", xtpBarPopup)
    Set Control = TempBar.Controls.Add(xtpControlButton, 35001, "Temporary")
    Control.Execute
End Function

Simply call this function and pass a reference to the CommandBars object you want to customize.  Enjoy!!