It's OK to freeze now.
You can add more milk to that bottle in the freezer, but only after the new milk has been well chilled in the fridge.
Defrost by leaving in the fridge or if you need faster defrosting then stand it in warm water and swirl every so often. If you need uber-fast defrosting then stand in water from a hot kettle - but the milk won't be as nutritious as if it were defrosted more gently.