Hi,
It's not possible to add a break line in the product name. The br tag is stripped automatically by the system if you try.
And that's normal since otherwise, you would also have the break line everywhere: on the checkout, in the order, the invoices, the emails and it would just look bad.
Now if you really want to do that, you can change the line:
$noFilter = array();
to:
$noFilter = array('product_name');
in the file administrator/components/com_hikashop/classes/field.php but I don't recommend it.