Hmm, a case of bad product selection on my side. I assumed (is the mother of all f..) that the difference between NAS and XSR was the software raid capability of XSR.
I have a new stategy; Create a RAID5 of 4 disks. During the migration create a second RAID5 of 4 disks.
The only drawback of this stategy is the loss of an extra disk.