Add a specific category at a specific place to the menu that uses wp_list_pages

wp_list_pages has limited functionality, and would be difficult to inject Categories into the mix without rewriting the function itself.

Using the built in Menu functionality is likely the best way to go, because you can at least set individual item classes, allowing you to create Menu headers for your top level categories (having the header items use a “header” class or similar).

You may also be able to just use the get_pages() function, which will return an array of the pages instead of just printing them onto the page. This will allow you add logic to the display of the data when iterating through the array.