Agree with Sammus, no need to freeze.
I would melt down your puck and pour it into a heat proof, covered flask of some sorts, about double the volume of your wort agar, and re-sterilise it in your pressure cooker. Once it has cooled down, make sure it is well sealed and chuck it in the fridge. When you need to pour some more plates or slants just melt your agar down again gently in a water bath (50-60 deg), which will minimise any condensation in your slant vessels. Just make sure you give your wort agar a good swirl to make sure the agar is evenly distributed throughout the media, and don't cap any of your vessels too tightly in the pressure cooker so any steam can vent.