You can't acclimatise the fish, it is far too late in the season, the shock to the temperature if they've been in an aquarium in a house all this time would make them seriously ill or kill them.
I would suggest keeping the fish in the tank they come from/or a large plastic tub in the house, for a little bit longer and introduce them to your pond when it's much warmer
But do make sure you aren't over stocking your pond by adding them!
Also as for the Koi just be sure to keep the water quality good, may I ask how deep your pond is? If it isn't deep enough to over winter them, are you sure it's deep enough for them generally?