Using either "add" or "pour" is acceptable. "Pour" is more specific (which you should consider if you are giving directions to someone), but I think most native speakers would use "add". If you begin the sentence with "add" it would sound best if you use "to" instead of "in" or "into", though neither is incorrect.
New words, one handy idiom, and a 2-minute quiz — delivered to your inbox to keep your streak alive.