Hi,
If you don't want registration, you can just turn off the new user account setting of the Joomla user manager.
If you want to add a new registration method you need to:
- edit the files "show_block_login" and "sub_block_login_registration" of the checkout view via the menu Display>Views.
- edit the files administrator/components/com_hikashop/classes/user.php and administrator/components/com_hikashop/helpers/checkout/login.php to handle the actions to take when someone click on the next/register button and validate the login view with your new checkout method.