You'd have to install a
winterization kit if you don't already have one.
The Rube Goldberg approach might then be to suck store bought water into your system via the newly installed winter kit until pressure is built up. Then use a hose connected to your outdoor shower to connect to your fresh water intake. Then keep pumping until you run out of one gallon bottled water.
A gallon of antifreeze gets emptied in under a minute I'd expect the same for water.