Hi,
If you want to remove the HikaShop registration form, you can edit the file "registration" of the view "user" (or "sub_block_login_registration" of the view "checkout" with the new checkout of HikaShop 3) and add such code that the top:
If you want to have instead a link to the CB registration form, you can add it before that line with such code:
<a href="http://mywebsite.com/link/to/CB/registration/page">Click here to register</a>
It's not possible to directly put the CB registration form inside the login view. Neither of the system is made to support that.