Question

I found a topic about how to add categories to a wordpress rss feed, but how is it possible to add a custom field to a wordpress RSS feed titel?

      function rssTitle_add_categories($title) {
    $category_array = array_map(create_function('$category', 'return $category->name;'), get_the_category());
    $categories = join(', ', $category_array);
    $title = $title . ' - '.$categories.' - ';
    return $title;
}
add_filter('the_title_rss', 'rssTitle_add_categories');

Answer 1

You just have to get your custom field using get_post_meta (or plugin API if you are using a custom fields plugin) and add it to your title, for example :

function rssTitle_add_categories($title) {
    global $wp_query;

    $category_array = array_map(create_function('$category', 'return $category->name;'), get_the_category());
    $categories = join(', ', $category_array);

    $field = get_post_meta($wp_query->post->ID, 'your_field_name', true);

    $title = $title.' - '.$categories.' - '.$field;
    return $title;
}
add_filter('the_title_rss', 'rssTitle_add_categories');