Show Joomla Username and ID on customer details

On the Order details page of the backend of HikaShop, it has the boxes Main Information, Additional Information, Customer, Billing address, shipping address. In the customer box it shows name, email and Id (this is the customerID in Hikashop).

I want to also see the Joomla username and Joomla ID. Is this possible? If so how? I can't work out which file in Display>View relates to that customer display box.

That's great thank Nicholas.... but what code do I need to add? to show Joomla username and Joomla ID

x

Here is code from my modification. It was done for Hikashop 2.6.1 but I think it should work also in Hikashop 2.6.2.
Add it at the very top of the file, right after defined('_JEXEC') or die('Restricted access');

if (!empty($this->order->customer->user_cms_id)) {

    $u = new JUser((int)$this->order->customer->user_cms_id);
    $this->extra_data['user'][] = array(
        'title' => 'Username',
        'data' => $u->username
    );
    $this->extra_data['user'][] = array(
        'title' => 'Joomla user ID',
        'data' => '<a href="index.php?option=com_users&view=user&layout=edit&id='.$u->id.'">'.$u->id.'</a>'
    );
}

It worked! Thanks so much