Yes hash brown casserole can be frozen and it will keep in the freezer for 6 months to a year.
You can either freeze the hash brown casserole before cooking it or after cooking the hash brown casserole.
If you freeze the hash brown casserole after baking it you should wait for 30 minutes or so to allow the hash brown casserole and baking dish to cool down.
You can freeze the hash brown directly in the baking dish if you want or transfer it to another freezer safe container and you should wrap the hash brown casserole with some plastic wrap or have a lid on the baking dish or other dish that you're freezing it in.
If the hash brown casserole has sour cream you can still freeze it but the sour cream will change texture when frozen and may not taste as good but it will still be safe to eat.