Is it possible to use the Tenable Add-On for Splunk for use with Nessus Professional without having Security Center? Our company has a license for Nessus Professional standalone, but doesn't have Security Center.
Yes, but only on certain versions.
There was a product change to Nessus Pro which removed the API for requesting reports in an effort to drive customers to SC or Tenable.io
There was some resistance to this by customers, and I believe that decision was revised in a later release. so it will very much depend on what version of Nessus Pro you are running,