Custom Feed URLs

I am creating a custom feed for one of my post types, and letting users set the name of that feed.

http://www.mysite.com/feed/userdefinedname

Now I’d like to also enable users to show not only all the posts from that CPT but also filter them by category. At the moment it works like so:

http://www.mysite.com/feed/testcategory

However I’d very much prefer it if I could structure it as follows:

http://www.mysite.com/feed/userdefinedname/testcategory

Is that possible?

Here’s the code that is generating these category based feeds:

 /**
 * Generate the feeds for categories
 *
 * @since 1.0
 */
function rss_c_add_category_feed( $in ) {

    $category = $GLOBALS['wp_rewrite']->feeds[ count( $GLOBALS['wp_rewrite']->feeds ) - 1 ];

    // Prepare the post query
    // Get published rss_feed posts with a rss_category slug in the taxonomy
    $rss_custom_feed_query = apply_filters(            
        'rss_custom_feed_query',
        array(
            'post_type'   => 'rss_feed', 
            'post_status' => 'publish',
            'cache_results' => false,   // disable caching
            'tax_query' => array(
                array(
                    'taxonomy' => 'rss_category',
                    'field'    => 'slug',
                    'terms'    => array( $category ),
                    'operator' => 'IN'
                )
            )
        )
    );

    // Submit the query to get latest feed items
    query_posts( $rss_custom_feed_query );

    $sources = array();
    while ( have_posts() ) {
        the_post();
        $sources[] = get_the_ID();
    }


    // Create the query array
    $pre_query = array(
        'post_type'      => 'rss_feed_item', 
        'post_status'    => 'publish',
        'cache_results'  => false,   // disable caching
        'meta_query'     => array(
                                array(
                                    'key'     => 'rss_feed_id',
                                    'value'   => $sources,
                                    'compare' => 'IN'
                                )
        )
    );

    // Get options
    $options = get_option( 'rss_settings_general' );
    if ( $options !== FALSE ) {
        // If options exist, get the limit
        $limit = $options['custom_feed_limit'];
        if ( $limit !== FALSE ) {
            // if limit exists, set the query limit
            $pre_query['posts_per_page'] = $limit;
        }
    }

    // query the posts
    query_posts( $pre_query );

    // Send content header and start ATOM output
    header('Content-Type: application/atom+xml');
    // Disabling caching
    header('Cache-Control: no-cache, no-store, must-revalidate'); // HTTP 1.1.
    header('Pragma: no-cache'); // HTTP 1.0.
    header('Expires: 0'); // Proxies.
    echo '<?xml version="1.0" encoding="' . get_option('blog_charset') . '"?' . '>';
    ?>
    <feed xmlns="http://www.w3.org/2005/Atom">
        <title type="text">Latest imported feed items on <?php bloginfo_rss('name'); ?></title>
        <?php
        // Start the Loop
        while ( have_posts() ) : the_post();

        ?>
        <entry>
            <title><![CDATA[<?php the_title_rss(); ?>]]></title>
            <link href="<?php the_permalink_rss(); ?>" />
            <published><?php echo get_post_time( 'Y-m-dTH:i:sZ' ); ?></published>
            <content type="html"><![CDATA[<?php the_content(); ?>]]></content>
        </entry>
        <?php
        // End of the Loop
        endwhile;
        ?>
    </feed>
    <?php
}