Hello. Yes, you can freeze fresh mushrooms without cooking them first, but you need to understand how their cellular structure responds to freezing.
Because mushrooms contain a very high percentage of water, freezing them raw causes the ice crystals to expand and break cell walls. When you eventually thaw them, they will release a lot of liquid and become quite spongy or soft.
If you still wish to freeze them raw to save time, make sure to clean them gently with a brush rather than washing them with water, slice them, and store them in heavy-duty freezer bags. They will work fine later in cooked dishes like soups, stews, or sauces, but they will not retain a fresh texture.