We can achieve this through batch process

I don’t think we need to write a plug-in in Jbilling to achieve this. This is as good as data export task from one db to another db irrespective of application (Jbilling / any thing.)

Develop a batch process which reads invoice details from Jbilling data base and update your desired database. Define a corn job or use a scheduler to run this periodically.

Please let me know more details with respect to your requirement.

Thanks,
Murali