Are you indexing any of your CMDB info in Splunk already? You can setup automated entity updates for ITSI, and an easy way to do this is with a Splunk SPL query on data already indexed. Here is more info in Docs.
If you don't have this sort of thing, a CSV-formatted file is a good starting point since that can already be used to manually update via the gui. Also keep in mind that if your CMDB can generate that csv for you, then you might as well index that file in Splunk (and you're back to the first paragraph) 😁
