Using links (link_rss field) as the source for fetch_feed

Possible, only I am not entirely sure about specifics of looping through multi-feed SimplePie object. Customize get_bookmarks() call to query for links you need.

$bookmarks = get_bookmarks();
$feeds = array();

foreach($bookmarks as $bookmark)
    if($bookmark->link_rss)
        $feeds[] = $bookmark->link_rss;

$items = fetch_feed($feeds);