Dropdown menu assist

While I appreciate that example code shows how to create a navigation bar
with menu items obtainted from the sitemap, I would like to add a second
level to at least one of the menu choices.

For example, my navigation bar now shows the following four choices for most
of my members:

Home....Safety Tips....Forums....Contact

What I would like to do is have the 'Forums' item present a drop down list
of various forums for my users to select from.

I cannot see how to get the data binding logic to go beyond the initital
selection while using the sitemap.

Is there a straightforward way to add a dropdown list to such a navigation
bar? Thank you.