I would like to add one entry for each user is the Level (CBUser) information.

To extend the user class and add more properties you shouldn't ideally need to make any change, instead you would use the property customData that is a Hashtable and you can store any type of data like Level. In the package you will find a sample implementation of the class DemoUser, you should create your own user class as it does.

PS: anyway if you're starting to integrate Combu in your project right now then I'd suggest you to use the latest 2.0 RC version of Combu, since it's the new system completely refactored. You can download the latest RC from Store > Purchase History, if you haven't it in the list and you purchased Combu from Asset Store then you can redeem your Unity invoice to be enabled on this site too.