In theory you can. All data is stored in one folder with subfolders and you can control where it is. It just needs to be mounted before server starts, which might be the most tricky part.
Regarding my experience with NAS I do not think this is a good solution. Also connected all with cable access is slow and sometimes OS hangs reading it and I need to reconnect it. Any such problems can cause problems during print adding extra delays you do not want or even stop print (if nas is not accessible when needed).
A better solution is to use Repetier-Server Monitor to backup server content from time to time. You can define backup folder how you like e.g. NAS. No timing problem, just ensure it is there when you make a backup. You can restore server with full backups if needed. Or select what to backup - only settings or projects or all printer data.
