I am not a DACS expert, but after reading your question - my understanding is that you want to populate the DACS entitlements database with third-party PEs so that you can then use DACS APIs/RTDS entitlement checking to control access to the 3rd party data.
If my above understanding is correct, then I believe this cannot be achieved using an API. I believe you would need to create a mapping file on the Dacs station/server and then perform a Map collect - which would then import those mappings into the DACS system.
For details on how to do the above and confirm my understanding. I recommend you create a support ticket for the 'Refinitiv Real-Time Data Access Control System' at My.Refinitiv. A DACS specialist should then be able to advise you further.