The simplle answer is, no. Coolant additives are meant to seal small cracks in a car's radiator. They may work for a brief amount of time in a car with a blown head gasket, but will soon leak again due to the extreme pressures bleeding from the cylinders into the water system.
The short answer is no, they do not. Head gaskets typically consist of materials like copper, steel, graphite and polymer coatings. When a head gasket fails it is due to deterioration of these components which creates a sealing void that no liquid sealant can replace.