Hi,
When you add a product in the cart, you can set the values for the product (also called item) ; so set a value for his custom fields.
But the address is a totally different thing. You can't change the address of the user like that.
If the user does not have an account, if you are using "guest checkout" for example ; it is possible to use the session to store an address which will be pre-filled during the checkout.
$addr = new stdClass();
$addr->address_firstname = '';
$_SESSION['hikashop_address_data'] = $addr;
Regards,
Jerome - Obsidev.com
HikaMarket & HikaSerial developer / HikaShop core dev team.
Also helping the HikaShop support team when having some time or couldn't sleep.
By the way, do not send me private message, use the "contact us" form instead.