Hello,
HikaShop allows you to have quantity per variant of a product.
And the pre-order products plugin allows you to have a "pre-order" quantity per variant too.
So when a variant quantity is 0, the product switch to "pre-order" mode and the customer have the message with a different button to add the product into the cart.
Regards,
Jerome - Obsidev.com
HikaMarket & HikaSerial developer / HikaShop core dev team.
Also helping the HikaShop support team when having some time or couldn't sleep.
By the way, do not send me private message, use the "contact us" form instead.