Hi, great question but there might be no great answer indeed 🙂
To populate CMDB from another source, ServiceNow recommends using IntegrationHub ETL or Import Sets.
Also, consider to use IDR following a recommendation from another post
I would suggest to start with IntegrationHub ETL store application, which integrates a data into the CMDB without compromising the integrity of the CMDB. However if CMDB of target instance is empty, you may also try to use Import Set.
Hope it helps
