[Dovecot] Question about managing multiple domains

[Sergio Moscoso]

Actually we have configured a server (postfix,dovecot,mysql) with multiple
domains, but the problem resides in that we don't want to login via POP3
using "user at domain" just using "user", because we'll migrate our actual
platform (Qmail) to dovecot. Our users are logging to their mail boxes,
using only user name.
We want to ask if that's posible to use a function like a "vipmap" in qmail,
that allow to map an IP address to a domain.
We're using the dovecot 1.0.7, the configuration shows as follows:

login_dir: /var/run/dovecot/login
login_executable(default): /usr/libexec/dovecot/imap-login
login_executable(imap): /usr/libexec/dovecot/imap-login
login_executable(pop3): /usr/libexec/dovecot/pop3-login
first_valid_uid: 100
mail_location: maildir:~/Maildir/
mail_executable(default): /usr/libexec/dovecot/imap
mail_executable(imap): /usr/libexec/dovecot/imap
mail_executable(pop3): /usr/libexec/dovecot/pop3
mail_plugin_dir(default): /usr/lib64/dovecot/imap
mail_plugin_dir(imap): /usr/lib64/dovecot/imap
mail_plugin_dir(pop3): /usr/lib64/dovecot/pop3
imap_client_workarounds(default): delay-newmail outlook-idle netscape-eoh
imap_client_workarounds(imap): delay-newmail outlook-idle netscape-eoh
imap_client_workarounds(pop3): outlook-idle
pop3_client_workarounds(default):
pop3_client_workarounds(imap):
pop3_client_workarounds(pop3): outlook-no-nuls oe-ns-eoh
auth default:
  username_format: %Lu
  passdb:
    driver: sql
    args: /etc/dovecot-mysql.conf
  userdb:
    driver: static
    args: uid=101 gid=102 home=/home/vmail/%d/%n

We'll apreciate your response, 'cause we coulnd't find information about
this issuet and we didn't to find any clue that shows how can realize it.

Thank you in advance.

Sergio Moscoso