As long as the QNAP can be accessed via something like a drive mapping (not recommended for internet use), FTP, SFTP, etc. you should be able to use it as a destination.
Most likely you’ll need to do port forwarding for whatever technology you decided to use for the destination (usually something like port 21 for FTP, port 22 for SFTP, etc.).
There seem to be a dozen or so posts about QNAP machines, some of which imply using destination technologies such as the ones below, so it’s definitely doable.
- SSH / SFTP @ SSH Host Disappeared (QNAP NAS)
- S3 @ QNAP with S3 Compatible
Perhaps @Kahomono or @marbletravis have some suggestions that could help get you started. If you do get it going, please consider posting how you did it here as a #howto to help other users out! 