How do I import multiple source systems and source system reference values for child objects?

Use Case: The CDM is currently importing its data from the source application via import management through a CSV file. The business is now transitioning to a new source application. While the transition is ongoing, we need to have both the old and new SS (source system) and SSRV (source system reference value) in existing records in CDM. We successfully added the new SS and SSRV to the existing records in parent objects (Accounts and Contacts). We are now on child objects, which are addresses, contact points, and relationships.

Current Activity: We are trying to add the new SS and SSRV to the Source System table for all of the child objects but can't achieve this. Below is our current header in the import file.