How I am adding features and resources to an existing chart that I don't control is to create a new helm chart, with the existing chart (your public chart) as a dependency. You can then add the new pvc.yaml helm template into the /templates folder of that new helm chart.

One common pitfall is that the values for the dependencies go into the <name-of-dependency>: subtree in the values.yaml of the new chart.

In ArgoCD, you then reference the new chart.