At a guess, I'd say we'd have to increase the salinity much beyond what the 1000 - 2000ppm most of us have been targeting.

I remember from college physics class that the absolute minimum freezing point of salt water is around -20° C. This is only at the point of maximum salinity in which the water can not possibly dissolve any more salt, which is somewhere around 20% salt. These are guesstimate numbers vaguely remembered through the fog of college labs and tests. I don't remember the exact values, but I'm sure I'm close.

Otherwise, any increase in the salinity of water will drop the freezing point by some amount, but I'm not sure if it is on a linear scale or not. Those of us who have added salt to our water will detect that our water will not freeze at 0°C, but maybe at -2°C or maybe a little warmer or colder. In other words, we haven't added enough water to truly make a difference in the freezing point, but we have lowered it somewhat, even if minimally.