Is it possible to create sub object pages similar to sub topics on navigation menu in the Napili template?
This is same as - Navigation Menu
This feature is delivered in the Spring'17 release:
Customize Community Navigation with Nested Menu Items
The new Menu Label type in the Navigation Menu component lets you create parent headings for your navigation menu and nest items underneath. Nested navigation menu
Nested navigation menu
Add a menu item, and set the type to Menu Label. Nest other menu items underneath by dragging them to the right under the menu label.