I would guess if they are a few years old now they are suitabely tough enough to go outside, provided the temp is fairly stable now (the deeper the water the more stable it will be). The trick with any outdoor pond is stablity, keeping birds from eating yuor fish and keeping them clean.
Also if the water quality is very different make sure to do a slow acclimation so they aren't shocked by a sudden temp change etc.....
With a pond that big adding a few more goldfish wouldn't be a bad idea.
Of course I am no goldfish expert so hopefully if I am in error someone else will jump in here.