We have this nice template for a K2item and we want to show related items by tag. It's a basic funtion but unlike the k2 tools module, we are not able to choose the category of the related items we want to show. Is there a possibility to ad the category ID in the item.php?

<?php
foreach($this->relatedItems as $key=>$item):
if($this->item->catid != $item->catid):
continue;
endif;
?>

This will work.

A trailing endforeach; is missing and I do not think you need continue;
Also you should use $this->item->category->id, to get the category's id.