Those are not rss feeds, they're json files. If you have PHP you can probably import and parse them fairly easily. Using javascript and jQuery, a similar thing can be done, except you still need need PHP or another server side language to get the file. At that point it would probably be easier to parse it with PHP or that other language as well.
Unless Google or some other large host has such a third party service, you would have to do it yourself if your host has PHP.
This:
https://developers.google.com/gdata/docs/json
may or may not be able to be adapted to grab those files so that javascript on your end could interpret them.
But, as I say, with PHP (requires a PHP enabled server that's allowed to fetch files from other domains) it's pretty easy, here's a basic example, save as newfiles.php:
<!DOCTYPE html>
<html>
<head>
<title>New Files</title>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
<style type="text/css">
.item {
border: 1px solid black;
margin: 3px;
float: left;
min-width: 225px;
font: normal 95% sans-serif;
padding: 2px;
background-color: lightyellow;
}
</style>
</head>
<body>
<pre><?php
$rawjson = file_get_contents('http://marketplace.envato.com/api/edge/new-files:themeforest,site-templates.json');
$arrjson = json_decode($rawjson, true);
//print_r($arrjson['new-files'][0]); // <-- this line may be uncommented to see the structure of the first entry
?></pre>
<?php
foreach($arrjson['new-files'] as $newfile){
echo "<div class='item'><a target='_blank' href='{$newfile['url']}'>{$newfile['item']}:<br><img src='{$newfile['thumbnail']}'></a></div>";
}
?>
</body>
</html>