It depends on what you add to the shakes. I wouldn't add fruit, milk, etc because you don't want to increase sugars and carbs since they're carefully regimented with medifast. However, depending on your nutritional needs and the condiments/healthy fats you do or do not use throughout the day, there's some room for manipulation.
For example, I don't use a bunch of condiments when I cook, so I normally have some to spare in that category. And a tbsp of peanut butter (I prefer the no sugar added kind but medifast doesn't say anything about it needing to be sugar-free) counts as a snack. Since I'm trying to up my protein intake and I'm not a snacker, it works out well when I add it to a shake.
Though typically, my shakes consist of just 1/2 ice and 1/2 water though - slushie power!