INN2 user authentication against system users

Hello INN2 list.
I'm a INN2 newbie user. :-)

I've setup INN2 on my VPS server with nnrpd nntps(563) port,
ckpasswd authenticating against a plain text file database,
my readers.conf fraction reads:

auth "aname" {
     require_ssl: true
                   ### format [email protected]:passwd_hash
     auth: "ckpasswd -f /etc/news/newsusers"
}

access "aname" {
    users: "*"
    newsgroups: "*"
    access: RPA
}

I've successfully authenticate against the newsusers file,
I tried to authenticate against system users but no success.
I don't want to maintain a seperate set of news users,
how do I do authentication against the system users,
like /etc/passwd and /etc/shadow?

The readers.conf man page is complicated, I don't quite understand.
Glad to get some example configulation. :-)

--
kevin