How to import the email messages ONLY from another account?

[Kent Brockman]

Hi, one customer will be transferring one domain to us from another provider, from which we nor he cannot access to the ssh permissions to transfer the full account. Although we have full ftp access to the /mail folders. There are a lot of email accounts, and also use maildir. So... Can we simply download them and upload it to the /mail folder inside their new account with us? Only need to run the following command:

/scripts/generate_maildirsize --verbose --allaccounts --force username

Is there anything else to have in account?

Thanks