SAS AML 8.x / VI 10.8 does not have a feature where you you upload/append records in bulk to internal entities through a pre-built user interface. The recommendation is to do so manually, using the front end investigator UI, although we acknowledge this can be cumbersome for bulk data uploads.
Whilst it is possible to write your scripts that directly write into the database schema's and tables, this is not recommended. If you do so, please consider the history and audit aspects of the data.
It should be possible to use the API for internal entity object creation and changes. SAS Professional Services should be able to help you out in this regard. For those who have access to the internal SAS pages, see Visual Investigator REST API – Visual Investigator API Reference (sas.com)
Specifically with regard to your question around the ID, in my experience it does not matter and you can use your own mechanism to define the key, as long as you make sure these values are unique. Not being an Postgres expert myself, I assume it has an auto-increment function that you can use, but in my experience you can also insert your own values as long as you keep them unique.