It is possible!
Years ago I did a 30 day carrot juice fast followed by 8 days of pineapple juice fasting and I gained 2 kgs, I think this is about 5 pounds. I gained on the carrot juice not the pineapple juice. As I was saying a couple of posts back weight is about more than just what enters your mouth and how much exercise you do. I don't fully understand the whole thing, but I know MH is right, its all about circulation. Look at these people (I know a lot of them, particularly men pre-40s) who can eat a whole pizza and fries and burgers any time, all the time and they are skinny!! Of course it will all catch up with them in the end, but what I'm saying is that weight does not magically dissappear for everyone on a healthy human foods diet.
Also I will just add that I was having only 3 glasses of carrot juice a day on that fast.
And when I gained 21 kgs (this is a lot of weight, maybe 40 pounds?? not sure) in 9 months, I was eating less than ever and all raw! And exercising a lot, trying to lose weight or at least stop the gain, but nothing worked, my thyroid and adrenals were crashing hard, and because my body had been raw for so long, there was no great perk for my body to be getting raw foods at that time, it was used to it.