In category product list view you can create your own product collection
for example you want show only few products using product ID (products are not assigned to the category)
at the top of the list.phtml add below $_productCollection=$this->_getProductCollection();

$getData=array(product_id1,product_id2,....); //$getData contains product IDs

$_productCollection = Mage::getModel('catalog/product')->getCollection()->addAttributeToSelect('*')->addFieldToFilter('entity_id', array('in'=> $getData));

if you want show products from more categories in one category

$categorylist=array(categoryID_1,category_ID2)

$_productCollection = Mage::getModel('catalog/product')->getCollection()
->joinField('category_id', 'catalog/category_product', 'category_id', 'product_id = entity_id', null, 'left')
->addAttributeToSelect('*')->addAttributeToFilter('category_id', $categorylist);