The core Joomla registration is not meant to be embedded in another form (the one of the checkout).
That's technically impossible.
That's why we took the second best option which is to create our own registration form which is completely compatible with Joomla's.
I suppose that you want to do that because of your other issue:
www.hikashop.com/forum/customers/878953-...onf-emai.html#212541
If so, the solution you're looking for is not in redirecting the customer to the Joomla registration but in simply fixing the reason why the notification email is not sent.
Adding a redirect to the Joomla registration form is dead simple if you know a bit of HTML.
Just add such line:
<a href="http://mywebsite.com/link/to/my/joomla/registration/page">Register here</a>
But it's a crude solution as it will break the flow of the checkout for customers and that's the last thing you want.