Only add a new goldfish to your existing aquarium or pond if it has the required space your goldfish needs. If you overstock them, it will lead to poorer water quality and require you to perform maintenance more often. Get more information about this by reading the next beginners post, How to Clean a Goldfish Bowl.
Transfer a Goldfish from a Tank into a Pond
If you want to transfer an indoor fish outdoors, do it slowly over the course of a week or so. Slowly bring the water temp in the aquarium down to that of the outdoor garden pond. Mix in some of the pond water into your tank with the goldfish. The key here is to not shock them with rapid changes in water parameters. Yes goldfish can live in both warm or cold waters but any fast temperature change will likely kill them. Slowly mixing in the pond water will also help with the transfer. Read more on this topic in goldfish water temperature extremes.