If they are outside now, leave them outside. Maybe put wood chip around the root balls (as is normal in the nursury). If you put them inside a green house you might mess with there dormancy. That not good for a tree. If you can get them right into the ground where you want to plant them, even better. You might also want to think about protecting them from the wind with a mesh tarp or such between posts, no over them. I would watch buying the hemlock. You are in a region that has a problem with Hemlock Wolly Adelgid. If you plan on planting them, you must also plan on spraying them yearly if you want them to live. Also Hemlock Elongate scale? is another problem we have here.