There won’t be an easy way to do a one-to-one migration like that as the ID structure in MongoDB is different (ObjectIDs vs integers) and the relationship table structure is entirely different also.
You may need to implement some customized migrations scripts reading from the CSVs and rebuilding a new one in the PG format. There may also being some tooling out there but since we (Strapi) have never used child documents they may not work.
This is one of the main reasons we are dropping MongoDB support in the v4: