Can the PBS backup network storage?
for example i have proxmox cluster with ceph(proxmox based) and some other network storage mostly NFS based (external to proxmox)
There is the proxmox-backup-client. You could set up a Debian LXC/VM and install the proxmox-backup-client package (or use your PBS that already comes with it), mount your NFS shares there and then use the backup client to create a host backup, which can backup any folder or block device: https://pbs.proxmox.com/docs/backup-client.html