Adding fish to your pond in the winter isn't a great idea, as this is the period of time in which fish are in a state of hibernation, and may not have the fat stores necessary to survive. If your pond is already at or below 50 degrees F, then your fish have already begun to enter torpor, and it may be hard for new fish to acclimate. Every fish needs time to adjust and settle in their new home but pond fish such as Koi and outdoor Goldfish need to do so before hibernating. The acclimatisation process should take approximately a month, during which their eating patterns can be established before the cooler months take hold.
If you've already gotten your new fish, but things are too cold in your pond for them to be introduced, then it may be a good idea to keep them inside until things warm back up. Purchase a tank or pool and keep it in a insulated place like a garage to protect the fish until it's time to introduce them to the main pond. During this time, if your fish are in a state of torpor already, refrain from feeding them so you don't have to feed them through the winter.
