How to Render Post Types in RSS Feed?

Write a small custom plugin wich a function, that enhance the default query for feeds.

The example below add all custom post types via get_post_types()to the default feed.

// add custom post type to wp post-feed
add_action( 'request', 'fb_add_to_feed' );

// add to post-feed
function fb_add_to_feed ( $request ) {

    if ( isset( $request['feed'] ) && ! isset( $request['post_type'] ) ) {
        $request['post_type'] = get_post_types ( $args = array (
            'public'          => TRUE,
            'capability_type' => 'post'
        ) );
    }

    return $request;
}

If you will control the custom post types inside the feed, then define the post types in the array of the code below.

// add custom post type to wp post-feed
add_action( 'request', 'fb_add_to_feed' );

// add to feed
function fb_add_to_feed ( $request ) {

    if ( isset( $request['feed'] ) && ! isset( $request['post_type'] ) ) {
        $request['post_type'] = array( 
            'post', 'stippet', 'archive', 'movies'
        );
    }

    return $request;
}