Hi,
By registration, you mean address infomations ? If it is, the step "registration" is required unless you have another page for account creation and address setting.
To have a one page checkout, you have to edit the views "checkout / step" and "checkout / address", in the "address" view, you probably have to remove the "register" button.
And on the "step" view add the action of the "register" button in the "next" button actions.
By this way, clicking on "next" will submit the address form, then reload the same page and you have to submit the main form.
It require JavaScript and PHP knowledges.
This is not really a clean way of doing that, because the page will be reloaded 2 times.
The best way is to change the forms and the actions, calling a method which will submit the address form and the main form at one time.
It require to edit HikaShop core files.