How Can A Plugin Hook Itself To the End of Every Excerpt?

From the codex there are a couple of possible filters for you to try:

• get_the_excerpt
• the_excerpt – look at @Mayeenul’s link for usage suggestions

Personally I would try the first one with something like:

function fileterExcerpt($excerpt){
    if(has_excerpt()){
        $excerpt .= "Some extra text";
    }
    return $excerpt;
}
add_filter( 'get_the_excerpt', 'filterExcerpt' );

Hope this helps.