CSS Can one easily add submenus into an existing site?

Hi - I made a website using a freewebsitetemplate (template Aviation School). However, as more content is being added to the side I think the best way forward is to add submenus into the existing menus at the top. Will this be easy to do? (If so what do I need to research?)

As before the original template was Aviation School available on the following link.
https://freewebsitetemplates.com/preview/aviationschooltemplate/

Thank you very much for any comments.

 

The best way I know of is to use CSS drop down menus, this way you can add several layers.
http://cssmenumaker.com/
http://purecssmenu.com/
http://css3menu.com/index.html
http://www.mycssmenu.com/
http://cssmenu-generator.com/free-dropdown-css-menu.html
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Is this what you wanted
falcon