wp-rss2.php is deprecated. Feeds are currently generated by special templates in wp-includes folder, namely feed-rss2.php for RSS 2.0. Of course they shouldn’t be edited just as well, only mentioning this for context of where you need to look for template details.
the_excerpt_rss() function passes it’s output through filter of same name the_excerpt_rss. You can use this filter to replace native excerpt with whatever you want.
add_filter( 'the_excerpt_rss', 'custom_excerpt_rss' );
function custom_excerpt_rss( $output ) {
return 'something to replace excerpt with';
}
Also I am not sure from your description if your feed is set to show excerpts in Settings > Reading. Depending on that you might need to use the_content_feed filter instead.