Encontrei uma forma, sem eu precisar mexer no core do WP, basta adicionar criar uma página chamada Feed e associar ao arquivo com o nome custom-feed.php, dentro deste arquivo basta adicionar o seguinte código:
<?php
/*
Template Name: Custom Feed template
*/
$numposts = 10;
function livexp_rss_date( $timestamp = null ) {
$timestamp = ($timestamp==null) ? time() : $timestamp;
echo date(DATE_RSS, $timestamp);
}
function livexp_rss_text_limit($string, $length, $replacer = '...') {
$string = strip_tags($string);
if(strlen($string) > $length)
return (preg_match('/^(.*)\W.*$/', substr($string, 0, $length+1), $matches) ? $matches[1] : substr($string, 0, $length)) . $replacer;
return $string;
}
$posts = query_posts('showposts='.$numposts);
$lastpost = $numposts - 1;
echo '<?xml version="1.0" encoding="UTF-8"?>';
?>
<rss version="2.0"
xmlns:content="http://purl.org/rss/1.0/modules/content/"
xmlns:wfw="http://wellformedweb.org/CommentAPI/"
xmlns:dc="http://purl.org/dc/elements/1.1/"
xmlns:atom="http://www.w3.org/2005/Atom"
xmlns:sy="http://purl.org/rss/1.0/modules/syndication/"
xmlns:slash="http://purl.org/rss/1.0/modules/slash/"
>
<channel>
<title><?php bloginfo_rss('name'); wp_title_rss(); ?></title>
<link><?php bloginfo_rss('url') ?></link>
<description><?php bloginfo_rss('description') ?></description>
<lastBuildDate><?php echo mysql2date('D, d M Y H:i:s +0000', get_lastpostmodified('GMT'), false); ?></lastBuildDate>
<docs>http://backend.userland.com/rss092</docs>
<language><?php echo get_option('rss_language'); ?></language>
<?php do_action('rss_head'); ?>
<?php foreach ($posts as $post) { ?>
<item>
<title><?php echo get_the_title($post->ID); ?></title>
<link><?php echo get_permalink($post->ID); ?></link>
<description><?php echo '<![CDATA['.livexp_rss_text_limit($post->post_content, 500).'<p>Saiba mais: <a href="'.get_permalink($post->ID).'">'.get_the_title($post->ID).'</a></p>]]>'; ?></description>
<extra><![CDATA[<p><strong>Texto e formatação desejada:</strong> <?php echo get_post_meta($post->ID, "custom-field-personalizado", true); ?></p>]]></extra> <!-- É aqui que vai o código do Custom Field desejado ou desejados. //-->
<pubDate><?php livexp_rss_date( strtotime($post->post_date_gmt) ); ?></pubDate>
<guid><?php echo get_permalink($post->ID); ?></guid>
</item>
<?php } ?>
</channel>
</rss>