How can I feed back plugins I migrated for myself?

I agree it is quite confusing. I am not very clear about the process my self, as there (currently) doesn’t seem to be a way for anyone other than the “owner” of a plugin submit it to the store (volumio plugin submit).
As mentioned in Volumio 3 Plugins store issues and discussion - #26 by volumio

When you submit a plugin via volumio plugin submit you become the “owner”. This is different from the “author” of the plugin.

So it would seem what you want to do is currently not very easy to achieve? i.e Make changes/update another author’s work. You could submit a PR, but this doesn’t lead to the plugin being published to the new store unless the “owner” runs volumio plugin submit