NetCommunity will allow the user to enter any username they like, as long as it is not a duplicate in the system. If you want the user to follow a certain format, you can add a note on the New User Registration form indicating your desired naming convention or you can create the username on the users behalf.

How to edit the New User Registration page
How to add users into Blackbaud NetCommunity from The Education Edge using the Add Users Utility
How to add a new user in NetCommunity
How to bulk add users from The Raiser's Edge to NetCommunity