There are several options:

1) [the most common way]: Use Diameter Ro. Even though, Specs describe Ro for Online Charging, you can do offline charging, too. See the ims_ocs module for an charging server implementation.

2) [the standards way]: Use Diameter RF. The Rf-interface is the standard interface in IMS for offline charging. I've got a ims_client_rf module in my private repo, which I can share, if needed (not documented and heavily tested yet, that's why I haven't committed it yet)

3) [the Kamailio way]: Integrate the Kamailio acc module.

Thanks,

Carsten