DotNetNuke has supported navigation providers since 2005. The provider model is based off of interfaces, promoting a one-size-fits-all mentality. This can be both a blessing and a curse. The curse as it relates to the navigation providers is that it limits the features that the navigation provider's control can support.
For example, if a menu wanted a mechanism to support horizontal submenus, the provider model would have to be changed which expose it to all providers, which of course makes little sense for something like a treeview With the released of DotNetNuke 4.5 the ability to specify custom attributes to the navigation providers was added. This can be accomplished by adding the following to the skin's ascx file.
The following directive needs to be added at the top
<%@ Register TagPrefix="dnn" Namespace="DotNetNuke.UI.Skins" Assembly="DotNetNuke" %>
Then within your skin object you simply embed a CustomAttributes element, then specify one or more attributes.
<dnn:NAV ID="dnnNav" runat="server" ProviderName="DNNMenuNavigationProvider">
<dnn:CustomAttribute Name="SubMenuOrientation" Value="Horizontal"/>
How it works
This is accomplished by adding a Generics collection to the NavObjectBase (which is used by the Nav.ascx skin object).
<DesignerSerializationVisibility(DesignerSerializationVisibility.Content), PersistenceMode(PersistenceMode.InnerProperty)> _
Public ReadOnly Property CustomAttributes() As Generic.List(Of CustomAttribute)
The provider will then be able to enumerate these custom attributes and apply them to the underlying control.
For Each objAttr As DotNetNuke.UI.Skins.CustomAttribute In Me.CustomAttributes
Select Case objAttr.Name.ToLower
Me.Menu.SubMenuOrientation = DirectCast(System.Enum.Parse(Me.Menu.SubMenuOrientation.GetType, objAttr.Value), DotNetNuke.UI.WebControls.Orientation)