Ok I tried this simple PHP script and is working:
$password = crypt('password' , '$1$my@email$');
print $password . " is the CRYPT-MD5 version of mypassword<br>";
Basically I've copied the result of this script and paste into the dbispconfig database, and is working.
The problem now is to add this script to the Roundcube plugin.
I tried to add something like that:
$passwd = crypt(%p , %u);
$rcmail_config['password_query'] = 'UPDATE mail_user SET password=$passwd WHERE email=%u LIMIT 1';
but I've have an error (white screen).