how to create a submenu under Extra

HI ive got the following question, now we have a menu bar above the page containing a vew items. Underneeth extra i need a submenu item. How can i realize that ? here are some part the codes.

public class PortalMenuItemList: List
{
private PortalMenuItem _Owner = null;
public PortalMenuItemList(PortalMenuItem menuItem)
{
_Owner = menuItem;
}
public new void Add(PortalMenuItem menuItem)
{
if(menuItem.Parent!=null)
menuItem.Parent.Children.Remove(menuItem);
base.Add(menuItem);
menuItem.Parent = this._Owner;
}
/*
public PortalMenuItem GetCommandByName(string commandName)
{
foreach(PortalCommand command in this)
{
PortalCommand temp = command.GetCommandByName(commandName);
if (temp != null)
return temp;
}
return null;
}*/
}
}

public class PortalMenuItem
{
public string Target;
private PortalMenuItemList _Children;
public PortalMenuItem()
{
Visible = true;
_Children = new PortalMenuItemList(this);
}
public string Title { get; set; }
public PortalMenuItem Parent { get; set; }
public string Command { get; set; }
public string Class { get; set; }
public bool Visible { get; set; }
public Dictionary<string, string> CommandArguments { get; set; }
public PortalMenuItemList Children { get { return _Children; } }
}

public override void InitMenu()
{
if (UserLoggedIn)
{
var features = new Features(User);
switch (User.loginType)
{
case UserLoginType.User:
AddMenuItem(new PortalMenuItem
{
Command = ApplicationCommands.UserPhoneManagement,
Title = "Mijn Toestel",
Class = "phone",
Visible = (features.CallingDeliveryBlock || features.DoNotDisturb || features.CallForwardingAlways),
CommandArguments = new Dictionary
{
{"userId", User.username}
}
});
AddMenuItem(new PortalMenuItem
{
Command = ApplicationCommands.ReceptionistView,
Title = "Receptionist",
Class = "contacts",
Visible = features.Receptionist && (AuthorizeUser(User.username, null, ServicePacks.DeanReceptionistEnterprise) || AuthorizeUser(User.username, null, ServicePacks.DeanReceptionistSmallBusiness)),
Target = "_blank"
});
AddMenuItem(new PortalMenuItem
{
Command = ApplicationCommands.HelpPage,
Title = "Help",
Class = "help",
Visible = true
});
break;
case UserLoginType.Group:
case UserLoginType.Partner:
case UserLoginType.DeanOne:
AddMenuItem(new PortalMenuItem
{
Command = ApplicationCommands.UsersView,
Title = "Gebruikers",
Class = "phone",
Visible = features.ReadUsers
});
AddMenuItem(new PortalMenuItem
{
Command = ApplicationCommands.AutoAttendantsView,
Title = "Keuzemenu",
Class = "help",
Visible = features.AutoAttendentBasic
});
AddMenuItem(new PortalMenuItem
{
Command = ApplicationCommands.ExtraSettings,
Title = "Extra",
Class = "balloon",
});
break;
}
}
}

// Topmenu.aspx
<%@ Control Language="C#" AutoEventWireup="true" CodeBehind="TopMenu.ascx.cs" Inherits="OneXS.Orion.FrontOffice.Compleet.Portal.Controls.Topmenu" %>
<%@ Import Namespace="OneXS.Orion.FrontOffice.Compleet.PortalBase" %>
<%
if (Repeater1.Items.Count != 0)
{%>
"main_menu_wrap">
class="main_menu">
- "Repeater1" runat="server" EnableViewState="false">
- "PlaceHolder1" runat="server" Visible="<%# (Container.DataItem as PortalMenuItem).Visible %>">
- class="<%# ComputeCssClass(Container.DataItem as PortalMenuItem) %>">
- "<%#ComputeMenuitemDocumentUrl(Container.DataItem as PortalMenuItem)%>"
- title="<%#(Container.DataItem as PortalMenuItem).Title %>" target="<%#(Container.DataItem as PortalMenuItem).Target %>">
- class="icon icon-24 <%#(Container.DataItem as PortalMenuItem).Class %>">
- class="label"><%# ComputeMenuItemTitle(Container.DataItem as PortalMenuItem)%>

<% } %>