How can I add certain pages (not posts) to my sites RSS feed?

This is tested and works but you may get unexpected results.

function wpse_rss_pages($qv){
    $rss_feed = array('post', 'page');
    if(isset($qv['feed']) && !isset($qv['post_type']))
        $qv['post_type'] = $rss_feed;
    return $qv;
}

add_filter('request', 'wpse_rss_pages');

You could also seperate the feed by doing something like THIS.