Can lamination be added to cardstock that is already gloss, like the 14pt gloss cover?

Yes of course.. For example, 14pt gloss cardstock means that the paper has a gloss coated top surface so that what is printed on the surface will be appear shiny. People add lamination to their orders usually for one of two reasons They are looking for a much higher gloss level which is made by possible by applying gloss lamination. They are looking for extra durability and protection, which the plastic film based lamination also provides.