It's possible to create a sub menu inside your left side menu.
- First, visit your menus in the app customizer.
- Select your menu, or create a new one.
- Add a new WordPress/External Link menu item. Give it a url of #.
- Expand the item and add "submenu-parent" to the extra classes. Optionally add an icon.
- Add your submenu items below this item, and give them an extra class of "submenu-child"
That's it! Add as many child items as you like, then save.