You can create inside form of product page your input:
1- Add your text area in product Page
<textarea rows="4" cols="50" name="textOpt">
When you add the product in cart you can see the headers:
form_key:ITpMa08mBS3jpqyP
product:337
related_product:
qty:1
textOpt:test2
2- Extend Cart Controller
require_once 'Mage/Checkout/controllers/CartController.php';
class NameSpace_ModuleName_CartController extends Mage_Checkout_CartController {
public function addAction()
{
if (!$this->_validateFormKey()) {
$this->_goBack();
return;
}
$cart = $this->_getCart();
$params = $this->getRequest()->getParams();
try {
if (isset($params['qty'])) {
$filter = new Zend_Filter_LocalizedToNormalized(
array('locale' => Mage::app()->getLocale()->getLocaleCode())
);
$params['qty'] = $filter->filter($params['qty']);
}
$product = $this->_initProduct();
$related = $this->getRequest()->getParam('related_product');
/**
* Check product availability
*/
if (!$product) {
$this->_goBack();
return;
}
/****************/
/* Custom Params*/
/****************/
//Create custom option
$additionalOptions = array(array(
'code' => 'my_code',
'label' => 'This text is displayed through additional options',
'value' => $this->getRequest()->getParam('textOpt');
));
//Add Custom Option to product
$product->addOption($item->addOption(array(
'code' => 'additional_options',
'value' => serialize($additionalOptions),
));
/****************/
$cart->addProduct($product, $params);
if (!empty($related)) {
$cart->addProductsByIds(explode(',', $related));
}
$cart->save();
$this->_getSession()->setCartWasUpdated(true);
/**
* @todo remove wishlist observer processAddToCart
*/
Mage::dispatchEvent('checkout_cart_add_product_complete',
array('product' => $product, 'request' => $this->getRequest(), 'response' => $this->getResponse())
);
if (!$this->_getSession()->getNoCartRedirect(true)) {
if (!$cart->getQuote()->getHasError()) {
$message = $this->__('%s was added to your shopping cart.', Mage::helper('core')->escapeHtml($product->getName()));
$this->_getSession()->addSuccess($message);
}
$this->_goBack();
}
} catch (Mage_Core_Exception $e) {
if ($this->_getSession()->getUseNotice(true)) {
$this->_getSession()->addNotice(Mage::helper('core')->escapeHtml($e->getMessage()));
} else {
$messages = array_unique(explode("\n", $e->getMessage()));
foreach ($messages as $message) {
$this->_getSession()->addError(Mage::helper('core')->escapeHtml($message));
}
}
$url = $this->_getSession()->getRedirectUrl(true);
if ($url) {
$this->getResponse()->setRedirect($url);
} else {
$this->_redirectReferer(Mage::helper('checkout/cart')->getCartUrl());
}
} catch (Exception $e) {
$this->_getSession()->addException($e, $this->__('Cannot add the item to shopping cart.'));
Mage::logException($e);
$this->_goBack();
}
}
}
3- Verify Options
this modification will work on all products. You can validate that the parameter exists to prevent the options are created