Can you give horses sugar-free apple juice to increase their water intake?

You can certainly flavour the water with apple juice or something else palatable, perhaps with sugar rather than without to make it tastier. I do wonder why the horse is drinking poorly in winter. Is the water it gets very cold? If so, it would help to warm the water slightly. Water intake usually changes a great deal between a cold or frozen bucket and water kept at a milder temperature, for example in a heated drinker or an insulated water butt.