Yes—there are various ways to customize Kill Bill:
System properties—These let you configure the core platform. Also, most internal modules have a second, more advanced, layer of configuration, usually through XML files. These let you configure the dunning (overdue) policy, the various plans and billing policies, etc.
Plugins—If you need more advanced (billing or non-billing) logic, Kill Bill has a plugin capability. You can write custom code to extend or even override Kill Bill’s default behaviors. We can recommend development partners to help you do that, if necessary.