Hi,
I don't understand why the fact that the customer can add only the main product from the wishlist is a problem since they can also purchase the main product on the checkout if they delete the option from the cart there.
But anyways, if you don't want to allow for that, you should turn on the "group options with product" setting of the HikaShop configuration.
That way, the main product and the option will be seen as a single element and it won't be possible to modify them separatly in a cart or a wishlist.