Koi will almost always follow the biggest fish around. DON'T EVEN TRY TO ADD NEW KOI WITH THIS ONE. If you're using water from your main pond in the quarantine pond then just add 1 or 2 from the main pond with this one and move the bigger kohaku to the main pond. A koi should always have a QT buddy.
I would advise to do a PP dip on the big one before putting it into the main pond though.... This is just to get rid of any unwanted bacteria that might be present in the QT.