How to consume custom fields in RSS feeds
For this situation I prefer using the Services module, then enable a rest server and using the index. Here is an excerpt about it from the module's project page:


  A standardized solution of integrating external applications with Drupal. Service callbacks may be used with multiple interfaces like REST, XMLRPC, JSON, JSON-RPC, SOAP, AMF, etc. This allows a Drupal site to provide web services via multiple interfaces while using the same callback code.