Unfortunately it is not possible to add ATC buttons or an Add all to cart button in the body of the email.

The emails sent from Klaviyo or Mailchimp are delivered outside of Shopify. The buy now links contain each individual product URL and redirect the customer back to your Shopify store to purchase these.

A buy-all button or an Add to cart button would need to redirect customers to a cart page where products are already added to the cart and this can’t be done just by using HTML in the body of an email that is sent outside of Shopify.