List<Control> l = clickedButton.Form.FindMovableControlsBySource("S_CAT" + catNo);
if (l.Count > 0)
{
    MercatorUi.MovableControls.MovableListBox listBox = (MercatorUi.MovableControls.MovableListBox)l[0];
    listBox.AddItem(newCatId, newCatLib);
}