WA, you only have to do P1 long enough to get rid of the cravings: usually 3-5 days for most people. In the meantime, you could make the green smoothie with a fruit-flavored sugar-free syrup, perhaps.
You can certainly try having the smoothie as a snack and see if that helps. Different times of day help a lot. But berries have the very lowest GI of all fruit, so I'd be very, very surprised if they caused cravings. It's more likely that some sugar snuck into your diet. For instance, what's in the greens mix? Most of the ones I've seen have some sugar. You might want to investigate to see if sugar's sneaking in somewhere (you wouldn't believe the places I've found it hiding!
Either way, though, your life will be easier if you do P1 and get rid of those cravings. Fighting against it takes so much energy!