What packaged foods are you referring to? The ones that come to mind for me - cereal, for example - include nutritional content for just the cereal; milk is added over and above.
Do you mean instant mashed potatoes, or something similar? You could bring up the nutritional content of the milk recommended in the directions and compare it to your soy milk label. Reduce total nutrients by the difference. That's the easiest/only way I can think of. The differences will generally be sort of minimal, though. I'd just track it as-is, and know I've got a tiny cushion not reflected in the #s, but I'm lazy like that.
Starting: 41.1 BMI and extremely sedentary
Current: 28.0 BMI with strength-training and low-impact cardio
Mini-goal: 29.9 BMI (about 164 lb) - DONE on 8/6/14! I'm no longer obese!
Mini-goal: 5K walk or run
Mini-goal: 24.9 BMI (about 136 lb)
Mini-goal: half-marathon walk or run
GOAL: 23 BMI (about 125 pounds), fit and active
| current weight: 163.4