I have just been doing the same thing for about 40 users, and the easiest way for me was.
On target server:
I created all the email users in ISPCONFIG.
Then using WinSCP on a windows box, I copied over the vmail/maildir directory for each user from source server to target server. Or you could archive to a file the expand again on target server providing the directory structure is the same.
Then once the copy to target server is complete, set owner/group to vmail (5000) for all files and sub-directories in /var/vmail.
Sart dovecot on target server.
I didnt have the passwords for most of the users so i just dumped from source msql database and then imported into the new ISPCONFIG database.