Hi,
First, I would recommend to turn off the "checkout legacy" setting of your HikaShop configuration. That way, you'll be using the new checkout of HikaShop 3 which is fully AJAX and has been completely rewritten. Thanks to that, it now handles true one page checkout where you can have the finish button directly on the first page of the checkout.
So all you'll have to do left is to hide the "next" button of the registration form and you can do that with a bit of CSS:
#hikashop_register_form_button{ display:none; }
www.hikashop.com/support/documentation/1...ize-the-display.html