Not sure you can at this time.. Only categories..no subs
Ok but why are in the autodesk library subs?
Yes if you look in some of the libraries there is for example Metal:Aluminum (but its Metal/Aluminum) and when you try to enter a "/" or ":" it shows as an illegal character error.
So I don't know how you create subs like that.. I'd bet they were hard coded by Autodesk and its just another thing to add to the list of half finished functionality.