Hi,
You have to create a menu for the registration using the HikaShop registration (with address etc..) then in the module option, disable the option "link to the product page", display the add to cart button.
In the Main > Cart options, set the option "After a product is added to the cart" to "Always go to checkout".
By this way the customer will be redirected to the checkout, and in the checkout you have to add a little code in the view "checkout / step", check if the total price is equal to 0, if it is submit the form.
With that code, the checkout will be proceed automatically. Some code will be required too for the redirection, in the view "checkout / after_end" make a redirection to the home page.