Not sure how you would do this. Many "greens" don't freeze well (think of lettuce). The consistency would change, that's for sure. Not too sure how the chickens would like it.
If you want to give your chickens greens in the winter, just get a bale of leafy alfalfa hay. We have a small hay feeder inside the chicken pen and our chickens love the hay. They go through a surprising amount of it too.
I think you will find it more convenient to use finely shredded cabbage or to buy a sack of dried alfalfa chunks which can be softened with hot water...I suspect the grass will not thaw well and might go bad easily after that...
what about a place to grow some indoors? If you have a large and sunny enough space and a flat pan or something you might be able to grow near a window and just clip some every few days for a little treat.