Hi,
you can attach the NAS "manually" on the Proxmox Backup Server and then use that as datastore backing path.
We recommend using a local storage on the server, to avoid doubling the network load.
If you run a dedicated high speed connection directly from the PBS to the NAS you may be fine regarding resource usage, though.
But, we highly recommend keeping backup data and the software which manages it (PBS) closely together, to avoid coupling of the NAS and the PBS which may become a problem in an outage.