When I was living on nutrition drinks ( I used Resource 2.0, vanilla) I had a couple of different methods of adding flavor.

One was adding iced coffee syrup; there are a couple of products out there designed to mix with milk, and they worked well.

My favorite method was to add frozen fruit smoothie mix. they come in just the right size for blending with an 8 oz. nutritioin drink, and turn a pretty blah experience in to a nice milkshake. The bonus was that the smoothie mixes also added about 200 calories to the drink, so with the 500 from the Resource, made for a reall high-cal meal.
Wayne