It wouldn't take too long to knock a script like that up. With the Magpie RSS library, for instance:
class Article {
var $title;
var $date;
}
$articles = array();
$rss = fetch_rss("http://some.url");
foreach ($rss->items as $item) {
$article = new Article;
$article->title = $item['title'];
$article->date = parse_w3cdtf($item['dc']['date']);
$articles[] = article;
} I confess my PHP knowledge of classes is a little rusty, so there may be errors in my example, but I hope that gives a starting point. Once you have arrays of all your feeds, it should be relatively easy to combine and sort them.
Alternatively, use a RSS combining website to create a new combination RSS feed and integrate that into your site.