Quick and Dirty: add an entry with the name '__________________’ and no action.
Otherwise add styling via CSS using the selector “nht-child” adding for instance a bottom border.
Not within one navigation bar widget. However, you could edit your page layout to use multiple navigation bar widgets, each with their own menu. One of those menus will be the standard project navigation, and others would use menu documents that you add to your project (just like you add page or a microflow).