Stores can only use supported payment providers to accept payments in their stores. If the payment provider you hope to use is not listed there then it would not be possible to use them in your store. You can view a full list of supported payment providers for your region by selecting your country here.
If you can tell me what country you're operating in and tell me why Knet PG is the payment provider you wish to use then I'd be happy to add them as a feature request so our Products team. They can look into if we're able to work with KNET PG in the future to create a payment provider.
We are happy to assist you integrating Knet PG with your Shopify store. feel free to DM us for more details.