Cool and dry, those are the best storage conditions.
Freezing? I just don't know enough about low temperature affects on the physical state of gunpowder to know if it is good or bad. Certainly water condensation is bad. I have read of phase changes with gunpowder at very cold temperatures, such as minus 40 F. Polymers have these glass transition temperatures, which is why ironing cotton fabrics work, and gunpowder is a very complicated mix of components. I would believe that if you can keep the stuff above freezing that would extend the lifetime of your powder.
I would not put gunpowder in an non operational freezer. If it auto combusts, that freezer will allow the pressure curve to spike before the door blow open. Then will become a big, badda, boom!
Heck, its only fertilizer, now what bad can come from that?