Online external NAS drive backup

What (if any) online backup services are available for and compatible with a stand-alone NAS drive?

I currently use (and am very happy with) Carbonite for individual PC backups. Unfortunately, Carbonite only backs up the internal drive(s) of a PC; it will not back up a NAS drive.

I have a client who does medical billing who would like to do online NAS backup in the event of a fire or natural disaster. My questions are:

1. Is there an online backup service that will work directly with a NAS drive with FTP?
2. If not, what online backup programs installed on an individual PC will allow backup of a NAS drive (as opposed to only a single PC's internal drive)?
3. What are your experiences (good or bad) with specific services that support NAS online backups?

Thanks in advance for your advice.