Hi,
With HikaShop by default, customers cannot purchase products without any stock.
When you access a product page of a product without stock, you'll see that the "add to cart" button will be removed and instead, you'll see a "no stock" message.
I think that what you need is to add this plugin to your HikaShop:
www.hikashop.com/marketplace/product/109...rder-by-obsidev.html
That way, in each product, you can specific a "pre order stock". And when the customer reaches a product page without stock but still with "pre order stock", instead of the add to cart button, the customer will have a "pre order" button.
So then, you can use translation overrides to change the "no stock" message and the text of the "pre order" button to whatever you want:
www.hikashop.com/download/languages.html#modify