Add Custom Fields to Custom Post Type RSS

function add_custom_fields_to_rss() {
    if(get_post_type() == 'my_custom_post_type' && $my_meta_value = get_post_meta(get_the_ID(), 'my_meta_key', true)) {
        ?>
        <my_meta_value><?php echo $my_meta_value ?></my_meta_value>
        <?php
    }
}
add_action('rss2_item', 'add_custom_fields_to_rss');

You should be able to substitute and any other meta values you need to add to the feed.