You can't. This is a very delicate thing to do because ABMaterial is resposive. In the very beginning something like that was allowed, but since then the navigation bar has grown a lot it is not possible any more. You'll need to play with the 'HideOnMediumAndLarge' param of the top item and forsee an alternative for on a phone. It may be better to use a drop down in the topbar if you have to many top items.