Hi,
The modifications will probably be only in the "register" function of the file administrator/components/com_hikashop/classes/user.php
Maybe also in administrator/components/com_hikashop/helpers/checkout/login.php (but I'm not sure it would be necessary).
The ideal for a properly developed solution would be to develop a system plugin to instantiate an override class of the default class (look in the hikashop_get function of the file administrator/components/com_hikashop/helpers/helper.php for the specifics) extending from the default class. That way, you would be able to easily update your HikaShop, without having to reapply the changes each time.
I would recommend you contact a 3rd party developer. with these indications, a skilled PHP developer should have no problem doing it. Now if you're in a hurry, you might want to contact our partners since they are specialized in HikaShop customization and thus know better what to do.