How to set the post_status in template

Dears:

I just want to let some post expire after the expiration set in the custom field.

Now I am able to hide it showing from main loop, but all I want to is to set these posts to private instead of hiding them.

How could I write the code in the template?

<?php $expirationtime = get_post_custom_values('expiration');

 		if (is_array($expirationtime)) {
 			$expirestring = implode($expirationtime);
 		}

        $secondsbetween = strtotime($expirestring)-time();?>
		<?php if ( $secondsbetween > 0 ) : ?>
<!--=Article here========================================================-->
<!--=================================================================-->
		<?php else : ?>
		<h2>Expired</h2>
		<strong><?php wp_transition_post_status('private', '', the_post()); ?></strong>

		<?php endif; ?>
		<?php endwhile; ?>