Copy backup to plesk9 server and run:
/usr/local/psa/bin/pre9-backup-convert -v convert -d /var/lib/psa/dumps/  /root/plesk8-backup-file
then go to plesk 9 panel and restore the backup from the client and/or precreated domain.

How to restore plesk mail backup to cmail server . only solution for now is:
You need to separately back up/restore mail accounts on the Centralized mail servers using Plesk`s backup management tools.
For example, if you use Parallels Plesk for Unix as Centralized mail server, you need login to mail server using [...]

I had problems when using nfs storage to store virtuozzo backups . I don’t like to enable remote mount on startup time because network issues which could happen , and which can generate long startup delays.
so, if you mount nfs partition after node boots up, and virtuozzo already started you will not see backups in [...]