No, NFS would not be the best idea for a backup file.
However you could stop the array, copy the backup file to a faster device, and assemble the array again referring to the new backup file (if you have a faster device).
Going from 7 disk RAID5 to 8 disk RAID6 isn't really growing the array. The size stays the same. So everything has to be relocated in-place, so a backup file is needed the whole time.