o wait, thsi seems to work:
function fields_in_feed($content) {
if(is_feed()) {
$post_id = get_the_ID();
$output .= '<p>' . get_post_meta($post_id, "text", true) . '</p>';
$content = $content.$output;
}
return $content;
}
add_filter('the_content','fields_in_feed');