+1 on adding the lactose now. You can add fruit extracts at kegging or bottling.
Just curious why would you want to crash a weiss? If you want to crash I like to cool down to about 34 for 36-48 hours. You wont clear chill haze, but it will help yeast drop out of suspension...which to me you want in a weiss beer. Maybe Im misinterpreting your question.