[Dovecot] sendmail to postfix-dovecot

[Roger Berger]

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