I have a freezer full of pureed veggies. I buy fresh veggies, steam them, then puree in my blender. I bag them in 1 cup and/or 1/2 cup sizes in zip lock bags, label with a sharpie and freeze them. It only takes a minute to thaw the little baggie in the microwave and I can add them to just about anything.....just match the colors. Example, pureed carrots or sweet potatoes in pasta sauce. Pureed cauliflower in mashed potatoes, spinach in meatloaf, etc. It is a great way to sneak in extra veggies for my kids. I've even done pureed beets in pancake mix and called them Princess Pancakes because they are pink. The kids loved them!
Jessica Seinfeld has a great book out called "Desceptively Delicious" that explains the process.
I would think this would work the same for smoothies.