I'm afraid that you have to put it manually in the item.php file.
First you need to create a K2 template override (read here
getk2.org/documentation/tutorials/174-templating-with-k2-and-the-concepts-of-sub-templates how overrides work).
If you template already uses K2 overrides skip the first part.
Then open item.php, locate the
<?php if($this->item->params->get('itemTwitterButton',1) || $this->item->params->get('itemFacebookButton',1) || $this->item->params->get('itemGooglePlusOneButton',1)): ?>
<!-- Social sharing -->
.......
and right before the <?php endif; ?> add your pin it code button.
