This can be done by hacking the components/com_k2/models/itemlist.php file, at line: 955

add this line:

$rows[$key]->extra_fields = $model->getItemExtraFields($rows[$key]->extra_fields, $rows[$key]);


after this line:

$rows[$key] = $model->execPlugins($rows[$key], 'relatedByTag', '');

This way, you can use

<?php echo $item->extraFields->EXTRAFIELDALIASHERE->value; ?>

in the item.php template file to show extra fields for related items.