Shared PersistenceVolumeClaim(PVC) across namespaces

It looks impossible, just because PersistentVolumeClaim is a namespaced object. You can look detailed answer here: https://stackoverflow.com/a/35366775/9065705

If you are using a ReadWriteMany-capable volume (like NFS/EFS), you can create multiple Persistent Volumes (PV) pointing to the same NFS volume, one for each namespace where you want to create a PVC. They can all use the same NFS volume at the same path, or specify different subPath to constrain them to certain directories.