The billing itself is independent from the actual implementation details of those protocols used to carry out the call. PortaBilling communicates with the nodes on your VoIP network (gateways, gatekeepers, proxies) using the RADIUS protocol and the VoIP VSA extensions for it. Thus, from the point of view of PortaBilling, RADIUS requests related to SIP calls are nearly identical to those related to H323 calls (there are only a few different attributes).
PortaBilling is able to process both types of requests, so no special configuration in billing is required to support SIP or H323 calls. You only need to add your gateway to the system as a trusted node, and then both H323 and SIP calls reported from that gateway will be processed properly.
A node for the SIP server should therefore be created in your system so that it can communicate properly with the billing. Also, products that are to be allowed to use SIP services should have a rating entry that includes this node.