Yep, iSCSI is SCSI data inside an IP packet and runs over standard networking. I'd recommend a 1Gb minimum link between the QNAP and the Backup Exec server if possible.
I was going to explain how to set it up, but QNAP have done it for me: QNAP - Resources - Tutorials - How to create and use the iSCSI target service on the QNAP NAS
:)

