You can absolutely do that.
Just keep in mind that if you want to use a manual port forwarding, you absolutely must create a new port forwarding in your router, which uses a different external (WAN) port number than the one which you use with the QNAP server. (The internal (LAN) port number though must remain at the default 32400.)
See also the current method of claiming a Windows based Plex server: New Claiming Requirement for Windows
