Yep, no problems...just don't overfeed for a week or so.
If they're to stay out all winter, it might be worth wrapping the barrell in bubble wrap and putting some over the top too in very cold weather. Don't feed the fish in temperatures below 10 deg C
In fact, make sure that the barrel in not in the direct sun for too long. The water can heat up pretty quick in this weather and goldfish do not like warm water.