You need to use a design customization for the menu design area, and set a manual level. We do this on puc.edu. Take a look at http://www.puc.edu/academics and then click on the “Teaching & Learning Center” link on the left. The menu changes, and that link is now the top parent. For this, we use a customization, and set it to level 3.
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You apply it to the parent of the level you want to change.
So, in our case, the customization is applied to the "Teaching & Learning Center" asset, which has a few child pages.
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If you need it to be consistent across all your top level assets then it might be more manageable to apply it to the site asset and let it cascade then you would apply a customisation to the home page that prints the whole menu.