If you are comfortable working with HTML and Liquid, the functionality you're seeking can be created with a custom theme. First, take a look at our theme documentation. Alternatively, if you would like the support of a web designer certified to work in NationBuilder to create this functionality, take a look at our Architects directory.

You'll want to create a new file in the custom theme. You could call it _footer_nav.html and copy the contents from _nav.html into that file. The _nav.html file is how the top navigation bar is displayed. Then you can hardcode additional pages to display in your footer navigation in the _footer_nav.html file. 

Finally, update the layout.html file and add {% include "footer_nav" %} in the footer area of the file.