This will be a post 1.9.1 feature. 1.9.1 adds the <c:select/> tag (more correctly added in 1.9 but with a critical bug that is fixed in 1.9.1)

This RFE will extend the <c:select/> tag so that it includes an Add button that opens a pop-up window which allows the user to add credentials and when the pop-up closes then all the <c:select/> tags on the page will be refreshed from their doFillXYZItems callbacks.

The Add button will need to walk up the Ancestor in Path looking for credential stores that are writeable and that the current user has permission to write to in order to allow the user to select the store to write to.

The <c:select/> tag will perhaps need extension to allow passing the domain requirements so that the credentials can be added to appropriate domains... but as a MVP adding to the global domain will suffice.