What ever you add to one of these boats needs to be light! Don't fill it up with heavy basketballs or pool toys! Expanding foanm is a no-no because it has a tendency to absorb every drop of water that enters the hull, making the boat heavier and heavier.
Get a strong plastic bag, fill it with styrofoam balls, seal it up and then squeeze it through the inspection port and back past the centre case. Repeat the exercise till you are satisfied you have the reserve buoyancy you require.