Hi,
You have these triggers which are called each time a product is added or removed from the cart:
onAfterProductCheckQuantities(&$products, &$cart, $options)
onBeforeCartSave( &$cart, &$do)
onAfterCartSave( &$cart)
so you can implement a trigger in a plugin of the group "hikashop" and do the code you want there.
Beware of the fact that if you update the cart with the functions of class.cart while you are in these triggers, you might risk creating a loop which will crash the processing of the page.