dovecot - Nov 2013 - sendmail to postfix-dovecot

Hi,
I am a newbie to postfix and dovecot so I hope you can help me.
I have an old server running sendmail for many years now. On this server I have
about 100 domains and 500 users. The users are named as myuser or myuser.dom.
People use pop3 and imap for fetching reading their email. I've setup a new
server with postfix and dovecot2 on freebsd and managed to get it all working
with virtual users.
However, now I found out that I overlooked something; People have to login with
username at domain.tld<mailto:username at domain.tld> on the new server
but only use their username on the old server. This means they all have to
change their login credentials, which is not an option.
Is it possible to do some scripting before authentication where the username is
looked up in the database and the right domain is added to the account? If I
understand right it is not possible to use system and virtual users together.

Thanks,
Roger

Roger Berger skrev den 2013-11-27 11:41:
> If I understand right it is not possible to use system
> and virtual users together.
incorrect it is, see here http://wiki2.dovecot.org/VirtualUsers part of 
usernames and domains

its all about dovecot auth how its configured, if you want both system 
and virtual users make sure system users get domain stripped in auth

nned more help ?, post dovecot -n