You are right you can't install the raws on the NAS box. There is only 2 ways of backing up the NAS box.
1> Backup the CIFS shares on the NAS from the BE. You can add the ip address of the NAS in the user defined selection list and backup the CIFS shares. Make sure NDMP is disabled on NAS when you are doing the CIFS share backup.
2> Backup as device - you have to purchase the NDMP license and needs to be installed on BE server and have to enable the NDMP on the NAS. After putting the NDMP you will be able to see the volumes from BE and can do the backup of NAS.