I'm thinking this is not doable since no one responded. Well that's a shame for all of who are obsessive about our library management.
bunglemebaby gave you a good answer. I'll describe the process of adding a field and using it in a pane.
I use a Sub_genre tag I created myself using Tools/Options/Library & folders. I added values to that tag when I ripped my CDs.
I could have filled out the Sub_genre field later. Using MC 13's facilities to select files and sort them, I could add the values to a number of files at once.
I could then add a pane to any view I use by right clicking on the view name in the Tree of views under Audio. Choose "Customize View". In the dialog that appears, add a pane in the "Show categories in this order" area. Click on the Add button, select a field and then drag it up or down to change the order of the panes. (Top = leftmost, bottom = rightmost.)
You can also add new views so that you have a view with Genre, sub-genre and artist in addition to the one you now use without sub-genre.
What I have described can be found by exploring MC 13's menus and dialogs.
Bill