Updating RSS Feed

This one should be pretty simple, I just want to add some extra lines to an RSS feed for a specific category
(specifically, the website hosts podcasting, so I need extra lines in both the header and the post areas of the rss2 feed.)

I saw in the codex this:

remove_all_actions( 'do_feed_rss2' );
add_action( 'do_feed_rss2', 'acme_product_feed_rss2', 10, 1 );

function acme_product_feed_rss2( $for_comments ) {
    $rss_template = get_template_directory() . '/feeds/feed-acme_product-rss2.php';
    if( get_query_var( 'post_type' ) == 'acme_product' and file_exists( $rss_template ) )
        load_template( $rss_template );
    else
        do_feed_rss2( $for_comments ); // Call default function
}

Will this work? Where do I put it, in the theme’s functions file?

Please don’t recommend a plugin. I’d rather edit the theme directly if possible.