How to edit out-of-the-box recipient fields

Hello,

I would like to edit existing Recipient fields to, for example, change the label or force a choice of specific values with an enumeration.

In Campaign Classic this is quite easy, I can just create an extension and change anything I want by adding an attribute with the same name as the orinial field. Is this possible in Standard? I obviously can't edit the original recipient schema, and I can't add an attribute with the same identifier to an extension, because ACS automatically adds a prefix to it. Editing the xml of the extension's schema is also forbidden, even with sysadmin rights.

Can anyone help, or is this just impossible?

Thank you,

Xavier