As far as and OEM type retrofit, contact your local Grady dealer.
I don't see why the tank is not retrofittable, in case it leaks would have to be replaced (?)
Add just a washdown pump, some hoses, electric feed, fill and feed lines and fittings, and vent line and fitting. The fill fitting may be a combo fill and vent fitting too.
If you are a DIYer, go to a dealer who has your boat on display with the FW option, stick you head down back there and see where they fitted the tank, and where they located the fittings and hoses, take a camera and take photos, take notes, the dealer won't mind - ask first tell him why, then open yours up and take measurements, go to Tanks.com, for the polyethylene water tank, everything else at any marine parts supplier.
I'd say less 400 bucks DIY, 2-3x as much at dealer.
You should be able to do a final wash down of the boat for the day with less than a gallon of water, so a huge tank is not needed. When you pull the boat over to the gas station to get gas, water up at same time. Treat the FW water sys with non toxic antifreeze off season, one gallon will do it your case. If you cash and carry your gas, you can get a collapsible clear 5 gallon portable container used in camping to get the water onboard.
Good luck with the nice off season project.