User authentication using local file

I try to configure to have user authenticate to a local file called users, with their mailboxes in ../../spool/dovecot, not in their home directories. I have this working correctly with pam authentication on different servers(openSuse and macOS). This server has no pam authentication so I need to use a local file, but I can not make any sense of the documentation, can someone please help on which files I need to change and to which values. I tried several options but getting all kind of errors with UID.

Horst