ContextMenuOpening

 <Rectangle Fill="Purple" Width="200" Height="100" ContextMenuOpening="AddItemToCm">
      <Rectangle.ContextMenu>
        <ContextMenu>
          <MenuItem Header="Item1"/>
          <MenuItem Header="Item2"/>
          <MenuItem Header="Item3"/>
        </ContextMenu>
      </Rectangle.ContextMenu>
    </Rectangle>

  private void AddItemToCm(object sender, ContextMenuEventArgs e)
        {
            //check if Item4 is already there, this will probably run more than once
            var fe = e.Source as FrameworkElement;
            var cm = fe.ContextMenu;
            if (cm.Items.Cast<MenuItem>().Any(mi => (string) mi.Header == "Item4"))
            {
                return;
            }
            var mi4 = new MenuItem {Header = "Item4"};
            fe.ContextMenu.Items.Add(mi4);
        }