Yes, it's safe to freeze your HydraPak with a couple of caveats:
1. Please don't fill it all the way - ice expands and if it's completely full, it could split open the plastic.
2. Please be sure not to drop it, as an impact while at a low temperature can cause the plastic to split open.
3. When you remove your flask or reservoir from the freezer, add a little bit of cold water to it so that it can break up any sharp ice shards that might puncture the plastic film.
Otherwise, you're safe to freeze your flask or reservoir and enjoy ice cold water on your next adventure.