Assuming it's going to be in a system that's off for the night, and in a place with no heat like a garage.
There's already glycol in most pre-built cooling systems, and that's antifreeze basically. The more you add to water, the more it moves from being water that freezes at 32F. I've never seen one of these systems (Cooler Master, Corsair, Antec, CooliT, etc...) that shipped with just water in it. Most were blue actually, so it's not really water in itself and more than likely good to 0F. Noting that Glycol is already in the water, most systems have an ability to top them off still. So yes, you can add chemicals to them.
The Garage PC in Kansas saw some 20F nights and fired right up. Don't know if it'd freeze if not in a building and just sitting outside in the 0F air and wind...but it wasn't. The easiest cure here is to just leave it running. Moving water won't freeze, especially with a heat source.
Hmm...but then again, wouldn't an air cooler do well here?