is it possible to share storage from one node to another over the network?

There is no magic way how PVE can turn a local storage into a shared storage. You will have to use the CLI to set up a SMB/NFS server yourself and then add that to all nodes as a shared storage.

The PVE webUI doesn't provide any NAS functionalities. But PVE is based on Debian, so you can set that up yourself following some Debian tutorials.
 
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You could create a container, add the spare storage as a thick-hdd to the container and share it via some sharing protocol.

For example, Create an Ubuntu privileged container, add the spare storage on that node as a think hdd to the ubuntu container. Configure Ubuntu to share that storage as a NFS share. You can then connect as many proxmox instances as you like to the NFS share.