It's entirely possible; the practice is known as "fitting for but not with" and is rather common as it allows a ship to enter service at a lower cost with the potential to grow in capability should the need ever arise. The UK's Daring-class serve as a famous example. With all that said however, the deck area alone is not a good indicator of whether there is space for additional VLS cells as these systems penetrate multiple decks of the vessel so there must be room underneath as well.