Can the Add to Cart button be replaced with a link to an external site?

Hi,

Yes. There are two main things to do for that:
1. You want to remove the add to cart buttons for these products. (for example, a sale end date in the past, a sale start date in the far future, not stock, etc). You can customize the message that will be displayed instead of the add to cart button with a translation override:
www.hikashop.com/download/languages.html#modify
That way, you could potentially have instead such text "Please click on the button below to buy this product on Amazon" or no text at all.
2. You want to create a custom field of the table "product" with the "frontend" display setting activated and of the type "text" with the "input filtering" turned off so that in your products, you can enter the HTML that you want in there for the add to cart button coming from Amazon.
Alternatively, you can use a similar custom field of the type "link" so that you can provide only the URL and the text on the button in the custom field. This way, it might be easier to manage, but you'll want to add some custom CSS code to be able to have the link looks like a button:
www.hikashop.com/support/documentation/1...ize-the-display.html
stackoverflow.com/questions/8357058/how-...k-look-like-a-button