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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;
}

Now use the_excerpt(); to output the kind excerpt you want. All the best and let me know if it worked or not.