Hello,
You can use a view override for the backend view "subscription / listing".
In that view, you will find the content
if(!empty($row->username))
echo $row->username;
else
echo '<em>' . $row->subscription_user_id . '</em>';
which is dedicated to the display of the user ; you can add the display of the email address (in the same table column or in a new one) thanks to " user_email "
Regards,
Jerome - Obsidev.com
HikaMarket & HikaSerial developer / HikaShop core dev team.
Also helping the HikaShop support team when having some time or couldn't sleep.
By the way, do not send me private message, use the "contact us" form instead.