There are many ways you can achieve this. As you mention you want to do it for simple products. You can create your own controller and add products to cart.
You will need to pass product ids in hidden array and create your custom action to add product in cart.
<?php
class Custom_Module_AddController extends Mage_Core_Controller_Front_Action {
public function addProductAction() {
$products = $this->getRequest()->getParam('products');
$cart = Mage::getModel('checkout/cart');
$cart->init();
/* @var $pModel Mage_Catalog_Model_Product */
foreach ($products as $product_id) {
if ($product_id == '') {
continue;
}
$pModel = Mage::getModel('catalog/product')->load($product_id);
if ($pModel->getTypeId() == Mage_Catalog_Model_Product_Type::TYPE_SIMPLE) {
try {
$cart->addProduct($pModel, array('qty' => '1'));
}
catch (Exception $e) {
continue;
}
}
}
$cart->save();
$this->_redirect('checkout/cart');
}
}
?>
You can also pass qty in form with product array.