Add Attributes to Simple Product Magento

My client wants to add attributes to products that were imported as SIMPLE products - since he is a new Magento user – he did not understand (nor did I) the implications of adding products as SIMPLE products… and that's how we told him to do it since they *were* simple products at the time but he's now trying to improve the presentation by embellishing them with attributers… Were looking for a solution to convert the products.