Hi,
If the customer register with another method than the common or the HikaShop one that should not be a problem.
Once in the HikaShop checkout the customer will be able to login, and fill the missing data for HikaShop.
But, if the component/plugin used for the registration is not integrated with HikaShop it will not be displayed by default in the checkout registration process.
If you use a module, you can potentially edit the view "user / registration" (or registration_bootstrap) to add the code:
Which is a Joomla code allowing to display a module anywhere.
www.hikashop.com/support/documentation/1...ize-the-display.html