You can buy a scution cup like reverse funnel that will create a tight seal from the sea cock below the boat, and put the tube end into a bottle of marine/rv anti freeze, and run the pumps until it comes out the other end. I know plenty of guys who do this. I've never done it before, I run the systems dry and leave my sea cocks 1/2 open, you should NEVER store a boat out of the water with sea cocks fully closed or fully open, these 2 positions are the easiest for corrosion to lock them in place, being able to move the handle both directions will allow you to free it and keep it from breaking if necessary. Also, leaving it open allows all water to drip out of the system leaving it dry over time. Spray the sea cock from below and in the boat with WD-40 or a similar lubricant, last thing you want in the spring is a frozen sea cock that needs to be replaced.