If I understand you correctly, you want to use a Postgres database that runs outside of your DDEV setup, right?
I don’t think thats easy possible. But you can configure DDEV to use a Postgres database instead of MariaDB / MySQL. Please check the .ddev/config.yaml to see the available options.
After a ddev restart you should have a running Postgres database in your DDEV setup.
With ddev describe you can then see the database credentials. Normal user, password and host are db while port is the default one.
Hope that helps a little bit.