Yes! I haven't tried this yet, but it should be fine. The only thing to watch out for, is the zucchini will be wet and mushy after it's dethawed. So, a cup of thawed frozen grated zucchini is going to pack much more densely than freshly grated zucchini. Because of this, I would start by adding in 1/2 cup. If you would like more zucchini, then add in another 1/4 cup. To prep the frozen grated zucchini, I would rinse it under warm water to quickly thaw it out and then make sure to use a towel and squeeze out the excess water.