Submenus without Changing Page Structure

I predominantly use pages on my website and they appear on the menu. However some of the menus have gotten rather long, and I’d like to create extra submenus. However I cannot figure out how to do this without changing the page structure which will break a lot of the links.
It’s not really a big deal (I can live with the extra-long menus) but suggestions would be appreciated.
Thank you.