You can use plant-based milk. Full-fat soy and coconut milk are the better choices to create richer, creamier kefir. However, plant milks with low nutrient value will produce thinner kefir, and the taste can be a bit off. Some sources suggest refreshing your grains in a batch of animal milk once a week. I would stick to kombucha, jun, water kefir and wild sodas rather than retrofitting a ferment that thrives on animal-based lactose instead of plant-based sugars.