Out of luck @Robert H Weinstein, unless the model comes with it, it won’t work. They’ve not made it backward compatible. 

Dyson official line - Is the V15 Submarine head cross-compatible?

“Only those products sold with the Dyson Submarine wet roller head have the software needed to intelligently switch between wet and dry cleaning. Previous software is not compatible with wet floor cleaning.”

“We don’t currently sell the wet roller head separately as an accessory.”