[CentOS] postfix vurtual user

Hi,

Sory to bother you with my new-user questions.
i've got one redhat 9 box, acting as email server using sendmail. it
use local /etc/passwd.

last week i've just finished setting up a server, centos 4.4 as a
replacement mail server for the red hat 9.
i mainly refereing the tutorial at
http://www.howtoforge.com/virtual_users_postfix_courier_mailscanner_clamav_centos.

with some modification, it's now working. i installed postfixadmin as
it have freindly interface, because, beside me, other admin does not
feel comfortable inserting the data through dump black and white
command prompt.

there is 2 problems:

1. what should i do so that the user does not have to fill thier full
email address (user at domain1.com) at the login page of squirellmail.
it's moreindly to just put "user" as they used before.

2. how do i convert all the previous email to the new mailbox format.

anyone have had an experience doing this?
please help me. thanks.