The comments provided are pretty useful (create your own table for it\creating roles on the fly). In short, the functionality you are looking for is not currently available in the existing Universal Role Provider packaged with .NET. However, the URP is intended to only be a starting framework for you to work with\edit as you see fit. I would use a combination of these 2 suggestions, using the built in role provider tables to generate roles on the fly, and use a custom table to manage that hierarchy.
Hope this helps, and welcome to Stack Overflow!