Postfix uses the same mailbox format as Sendmail, so you can simply copy over your old mailboxes. Also copy the appropriate entries for these users from /etc/passwd and /etc/shadow and add their domains and email addresses to /etc/postfix/local-host-names and /etc/postfix/virtusertable. Run
Code:
postmap /etc/postfix/virtusertable
/etc/init.d/postfix restart
afterwards.