Yes, you can definitely freeze a CamelBak bladder without issues. Some people like to freeze their CamelBak bladders empty to kill any remaining bacteria in the pack after a day of use. Others like to freeze their water in the pack so it stays colder longer as it melts throughout the day. However, you have to make sure you don't fill the bottle all the way when doing this. Because water expands when frozen, your pack could become damaged if frozen while entirely full of water. Never fill it past 80% capacity if you plan to freeze.