Add US Zipcodes to existing Wordpress Database

We have a Wordpress site with 5000+ US Cities, each city being a page on the site. We need to match the zip codes from this database ([login to view URL]) to the corresponding cities on our site and add them to a custom field called 'zipcodes' (that does not yet exist for every page). Each page will likely have more than one zip code assigned to the zipcodes custom field. In such a case, zip codes need to be separated by a line break, so when viewed within Wordpress they are each on their own line in the custom field data box. This project should be able to be completed programmatically, ideally via a MySQL query.