Hey Faisal,
If you're a customer of Modernbill, you're more likely to be able to convince them to work with us than we are. And since the code changes have to happen on their end, we need them to work with us for it to happen.
Drop them an email and let them know you'd like to use Modernbill to manage provisioning of Virtualmin servers. You can also point them to our APIs:
Command line:
http://www.virtualmin.com/support/documentation/virtualmin-commands/Remote:
http://www.virtualmin.com/support/documentation/virtualmin-remote/And let them know that they can contact us for a developer license and with any questions they might have. If they need additional API functionality, we're happy to add that as well. The API is now in use by two billing apps (AgileBill, which you mentioned, and AWBS) and so it probably has everything a billing system needs, but if developers need something else we're generally happy to add it.