I suppose it could be done through an add-on package, however, I would question the advisability of that.
The DNS-323 itself does not need an anti-virus, the only way to infect it would be by flashing it with a corrupt firmware - also - the majority of common viruses are written targetting Windows based systems and do not attack the linux OS running on the DNS-323, and whilst viruses targetting linux do exist, they target the x86 & compatible processors, not the arm one used here.
It is certainly possible to store virus infected data on the NAS, but, rather than attempt to scan and clean those with the relatively low powered processor, this would be better done by the more powerful processor in the computer storing or retreiving the data.