Nas - did you mean should not stop postfix, or need not stop postfix? I thought of it not as importing, but restoring from backup. I did find a lot of hidden files & folders that also needed their ownership changed. Since NethServer was using bash, I used "shopt -s dotglob" before the chown command, and "shopt -u dotglob" after.
Path differences:
Zentyal 3.2: /var/vmail/domainname.com/username/ (folders cur, new, tmp) plus lots of files, some with "dovecot" in the name.
NethServer 6.7 /var/lib/nethserver/vmail/username/Maildir/ approximately same folders & files as Zentyal
So NethServer path does NOT include the domain name, and has extra folder Maildir