iRedMail ships a shell script for bulk adding mail users: iRedMail-0.8.7/tools/create_mail_user_SQL.sh.
If you have password as second field in each line of this text file, please update 'tools/create_mail_user_SQL.sh' with below steps:

1: Find below lines in 'tools/create_mail_user_SQL.sh':

DEFAULT_PASSWD='88888888'                                                       
USE_DEFAULT_PASSWD='NO'

echo '' > ${SQL}

2: Change them to:

DEFAULT_PASSWD="$3"   # WARNING: Must use double quote ("), not single quote (').
USE_DEFAULT_PASSWD='YES'

echo '' >> ${SQL}     # Two '>'

3: Remove '@domain.ltd' in your TXT file. For example:

- Original line: "user@mydomain.com     Dj(pZ3054"
- After modified: "user     Dj(pZ3054"

4: Execute this script with your txt file:

# cd iRedMail-0.8.7/tools/
# bash create_mail_user_SQL.sh YOUR_DOMAIN_NAME $(awk '{print $1,$2}' YOUR_TXT_FILE)

It will generate a new file 'output.sql' under current directory, please verify it first, if it's ok then import it into SQL server. NOTE: you need to create domain first manually.

My mistake. Please modify below lines based on the one you already modified.

1: find below lines inside function 'generate_sql()':

generate_sql()
{
    DOMAIN="$1"
    shift 1

    for i in $@; do
        username="$i"

        [...snip...]

    done
}

2: Update them to below one:

generate_sql()                                                                     
{                                                                                  
    # Get domain name.                                                             
    DOMAIN="$1"                                                                                                    
    username="$2"                                                                  
                                                                                   
    #for i in $@; do                                                               
    #    username="$i"

    [...snip...]

    #done
}

Then execute command again.

Hm, after I did the last changes, it only process the first line in my user/password file.

This command gave me the same result as the first try: the script generates a user based on the password.

What changes do I need to do in the script before the shell command?

Only this one?
"
DEFAULT_PASSWD="$3"   # WARNING: Must use double quote ("), not single quote (').
USE_DEFAULT_PASSWD='YES'

echo '' >> ${SQL}     # Two '>'
"

I got it working now, I got the output.sql and imported it, I`m now able to see the users in iRedMail admin, but the users are not able to log in through Roundcube, from /var/log/mail.log I get:
May 24 20:13:01 post roundcube: IMAP Error: Login failed for firstname.lastname@mydomain.com from MY.IP.ADD.RESS. LOGIN: Authentication failed. in /usr/share/apache2/roundcubemail-1.0.1/program/lib/Roundcube/rcube_imap.php on line 184 (POST /mail/?_task=login?_task=login&_action=login)

Could it be something with the hashing of the password?

It seems to be a password mismatch, I have duble checked that I have the correct password (a bit strange since the hashing seems to be the same?)

From my TXT input file used to generate password: firstname.lastname    Ks*eJ7610
From the output.sql:  VALUES ('firstname.lastname@mydomain.com', '$1$a76EIy2o$cTIB.SDOWWaOeSjky1ZlZ.', 'firstname.lastname',

From the dovecot.log:
May 25 13:43:27 auth-worker(13902): Debug: sql(firstname.lastname@mydomain.com,127.0.0.1): query: SELECT password FROM mailbox WHERE username='firstname.lastname@mydomain.com' AND active='1'
May 25 13:43:27 auth-worker(13902): Info: sql(firstname.lastname@mydomain.com,127.0.0.1): Password mismatch (given password: Ks*eJ7610)
May 25 13:43:27 auth-worker(13902): Debug: sql(firstname.lastname@mydomain.com,127.0.0.1): CRYPT(Ks*eJ7610) != '$1$a76EIy2o$cTIB.SDOWWaOeSjky1ZlZ.'
May 25 13:43:29 auth: Debug: client out: FAIL    1    user=firstname.lastname@mydomain.com
May 25 13:43:29 imap-login: Info: Disconnected (auth failed, 1 attempts in 2 secs): user=<firstname.lastname@mydomain.com>, method=PLAIN, rip=127.0.0.1, lip=127.0.0.1, secured, session=<4pKb+Df6RwB/AAAB>
May 25 13:44:40 imap-login: Info: Disconnected (disconnected before greeting, waited 0 secs): user=<>, rip=XX.XX.XX.66, lip=192.168.18.20, session=<mQnv/Df6sQBYVD5C>
May 25 13:44:40 auth: Debug: Loading modules from directory: /usr/lib/dovecot/modules/auth
May 25 13:44:40 auth: Debug: Module loaded: /usr/lib/dovecot/modules/auth/libdriver_mysql.so
May 25 13:44:40 auth: Debug: passwd-file /etc/dovecot/dovecot-master-users-password: Read 0 users in 0 secs
May 25 13:44:40 pop3-login: Info: Disconnected (no auth attempts in 0 secs): user=<>, rip=XX.XX.XX.66, lip=192.168.18.20, session=<ypDv/Df6sgBYVD5C>
May 25 13:44:40 auth: Debug: auth client connected (pid=13919)

Where do I change that?

I have added {MD5-CRYPT} in front of the user password via phpMyAdmin , but I am still not able to login:

Jun 01 13:28:34 auth-worker(6161): Debug: sql(firstname.lastname@mydomain.com,127.0.0.1): query: SELECT password FROM mailbox WHERE username='firstname.lastname@mydomain.com' AND active='1'
Jun 01 13:28:34 auth-worker(6161): Info: sql(firstname.lastname@mydomain.com,127.0.0.1): Password mismatch (given password: Ks*eJ7610)
Jun 01 13:28:34 auth-worker(6161): Debug: sql(firstname.lastname@mydomain.com,127.0.0.1): MD5-CRYPT(Ks*eJ7610) != '$1$a76EIy2o$cTIB.SDOWWaOeSjky1ZlZ.'
Jun 01 13:28:37 auth: Debug: client out: FAIL   1       user=firstname.lastname@mydomain.com
Jun 01 13:28:37 imap-login: Info: Disconnected (auth failed, 1 attempts in 3 secs): user=<firstname.lastname@mydomain.com>, method=PLAIN, rip=127.0.0.1, lip=127.0.0.1, secured, session=<cdxFlMT61AB/AAAB>

Is it possible to use the "CRYPT" hash when generating users instead of "MD5-CRYPT" (to prevente the mismatch)? The server is a VM so I am able to roll-back and import the users again...