Hi,
The login and registration are both included on the "login" step of your checkout workflow so in the same page, to do what you want you'll have to use some code customization which will require some development skills.
Also note that a solution can be to :
- Use the "Display method for registration" option through "Hikashop->System->Configuration->Checkout" and set it to "Switcher"
OR
- Directly edit the "login_form" file of the "checkout" view of your front-end template through "Hikashop->Display->Views"
- Add a link to your registration form.