Hello Chris Wilper,
Newsletters module is not GenericContent based module. It does not support dynamic metakeys so that the only way to create a new field and persist
it ( you can easily add a control with zip codes but it cannot be persisted) will require developing a new data provider. If the subscribers are CMS users
you can use Profile properties to persist Zip code data. Then you can use ProfileBase.GetPropertyValue Method
to show the ZIP code.
Let us know if there is anything else that we can do for you.
the Telerik team