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;
            }