I’d like to let the Developers respond directly to your request, but my assumption is that it’s not possible because Sub Categories are Item Attributes, rather than children of the navigation structure.
That said, a different solution is available through Department Grouping. This provides 3 unique levels of navigation; Department Groups, Departments, and Categories. Changing your navigation to use department grouping takes a few hours, but will achieve your goals. Here’s a tutorial on how it works - https://www.youtube.com/watch?v=CCXZbSArSWg