Appending a variable to RSS link url

add the following to your functions.php file:

add_filter('feed_link','my_site_feed', 10, 2);
function my_site_feed($url, $feed_type ){
    $user = my_get_the_username(); //this function should return the username.
    return add_query_arg( array('user' => $user ), $url );
}

And get your rss feed url using:

echo get_bloginfo('rss2_url');

or

bloginfo('rss2_url');