>>>Wouldn't the freezer bring some humidity and ruine eventually the hatching rate?
Moisture can get into the can whenever you open the lid. In the freezer, the moisture will freeze (i.e., frost) but will have no serious effect upon the cysts. Except when you remove the can from the freezer and "defrost" it. Many hobbyists will leave the can in the freezer and occasionally scoop out enough cysts to last a week or two rather than opening the can every day and introducing new moisture.