In my case, I setup a ZFS pool of my disks in my old desktop PC running Proxmox. Then I allocated some storage to an LXC container running Debian and Samba for file sharing.

In your case, since the QNAP already runs Samba, it would be best to run it directly on the NAS.

But if you want to do it for the learning experience, you can setup an NFS share on the QNAP and link it to the Proxmox. The Proxmox can then use the NAS for storage and you can have VMs or LXC contsiners use for virtual disks.