You can achieve this by using the_excerpt filter. You can read further on the codex. Just  paste this code in functions.php and set the value of $i = no of words from the end you want.


    add_filter('the_excerpt','my_excerpt');
    function my_excerpt(){
        global $post;                         
        $excerpt=get_the_excerpt();
        $content = get_the_content(); //gets the whole content                 
        $content = explode(" ", $content); //stores each word in an array
        $size = count($content);        //counts the length of array                                
        $last="";                       //initialize an empty sting
        $i=4;                           //no of words from the last you want
        while($i>0){
            $last.= $content[$size-$i];   
            $last.=" ";
            $i--;  
        }  
        return $excerpt."......".$last;
    }