[Dovecot] proxy user wildcard

Hi,

I'd like to try dovecot as a imapproxy in front of a webmailer. Authentication should be done by the main dovecot-imapserver. It works if I specify the user in passdb passwd-file:

user2::0:0::::proxy=yes host=[ip-address] ssl=any-cert starttls=yes nopassword=y

Is there some kind of catch-all function to pass all users to the main dovecot-imapserver without knowing the valid users?

dovecot: 1.2.9

thx Martin