Hi,
Not sure but I think that the sidebar open and close is done via Javascript.
So some javascript code could be added to the page in order to open the sidebar (so see with the template provider how to do that) when the "add to cart" button is pressed.
With jQuery that could be something like that:
jQuery('.hikashop_cart_button').click(function(){
// open the canvas sidebar
});
This code is very generic and will be call every time the customer click on an add to cart button.
That can be more specific by calling a JS function in a plugin based on the function "onBeforeCartUpdate()".
This require Javascript knowledges and in the second case PHP too.
Here is the developer documentation for the plugin part:
www.hikashop.com/support/support/documen...r-documentation.html