What Mohamed told you to do is to redirect other registration pages to hikaShop's. We don't provide any way to use another registration system that HikaShop's during the checkout.
The only solution is to add a line like:
<?php return; ?>
at the beginning of the file "login" of the view "checkout" via the menu Display>Views in order to not display the login/registration of HikaShop during the checkout and then add some code before that to display you registration form in there, or another way to display your registration form about the checkout form of HikaShop, like for example with a module.