Customizing RSS information

Hi,
I want my wordpress RSS to show other information (category name) besides the default title, link etc.
Is there anyway to do this?

I tried editing my wp-includes/feed-rss2.php like the following.

$cat_name = "";
$cats = get_the_category();
foreach($cats as $cat){
    $cat_name .= "$cat->cat_name ";
}
?>
<cname><?php echo $cat_name; ?></cname>

And tried to display the value with JavaScript.

for(var i=0; i<rss.feed.entries.length; i++){
   var entry = rss.feed.entries[i];
   document.write(entry.title);
   feedHtml += '<li>';
   feedHtml += '<a href="' + entry.link + '">' + entry.cname + entry.title + '</a></li>';
}

It failes, and i cant see any <cname> in my rss feeds and of course i go the undefined error when i tried to display entry.cname.

I bet i’m doing it totally wrong.

Can someone please enlighten me on how to add additional information to the RSS? (in my case, category name)

Thank you.