It's easy to create submenu items in Moodle, and the good news is that Edumy Premium LMS theme supports sub-menus and sub-sub-menus (and sub-sub-sub menus) from version 1.3.0 onwards.

To add submenus in your Moodle site, you can do so in the same way as you would normally add submenu items to the top-level menu items.

Just follow Moodle's standard menu item-level convention, as demonstrated here:

In Edumy, this should appear like so: