(I don't have one but) QNAP apparently uses Linux, and presents as "Software RAID" using standard "mdadm".
DO A BITCOPY BACKUP OF YOUR DISKS BEFORE TRYING THE BELOW - THERE IS ALWAYS RISK WHEN PLAYING WITH BLOCK DEVICES.
This means you should be able to boot a Pendrive Linux distro (Ubuntu Desktop can do this ), and then recreate the RAID array using MDADM with minimal risk of reinitialising the drives. You can Google authorative information on how to do this, but maybe try something like :
mdadm -A /dev/md0 /dev/sdxX /dev/sdyX mkdir /tmpmountpoint mount /dev/md0 /tmpmountpoint
Then copy the desired data from /tmpmountpoint.