Hi Rahman,
I wonder if you want to add DID to a2billing from an external web interface, or have an extension in a2billing that retrieves DID list from external source INSIDE a2billing DID interface.
Let me know so I know whether I have the solution for you. Normally the former is easier, since we can add codes in that web interface to deal directly with mySQL a2billing database
Thanks and regards,
Bao Nguyen