Sendmail Setup : Sendmail (mbox format) + Dovecot + Squirrelmail

I like to migrate a Sendmail Mail Server to iRedMail. I've isolated the usernames from the "/etc/passwd" file and stored them into a text file. (Total no.of users > 500)

Similar to this,
[root@mail ~]# cat user.txt
user1
user2
user3

My plan is to initially set each user's usernames as their password, which they can latter change through the Roundcube interface. For this purpose I intend on using the "create_mail_user_MySQL.sh" script supplied with iRedMail.

# sh create_mail_user_MySQL.sh domain.ltd newuser [newuser2 newuser3 ...]

Because of the large no.of users ( > 500 users) it would be impossible for me to supply each username at the command prompt. My question is will it be possible to make the script loop through the text file line by line to create the appropriate sql file through shell scripting. Thanks in advance. Other ideas for doing a migration are also welcome.