I don't see any way to do this in iTunes Connect. You get a list of primary categories, and then a second identical list for a secondary (not sub) category.
How are these subcategories determined?
How are subcategories in App Store decided?
Sub categories are chosen by you. I have apps in Utility -> business and apps in
Utility -> Entertainment
No. See reply to previous.
Those are strictly curated by Apple. The metrics are not known to the public, but as always, making an attractive app the showcases the platform is a good leg up towards that goal.