Appropriate to offer breastmilk in training cup?

My little guy is still very much in the playing/learning category with solid foods and has yet to ingest more than crumb-sized amounts of food yet. So I am not yet offering him water.

But is it appropriate to offer some of my milk in his little training cup? Or would this prevent him from taking to water as easily later, since he might get used to drinking something so sweet from the same cup?

Interested in your experiences!