mice.impute.passive(data, func)formula specifying the transformations on datamice 
  Gibbs sampling algorithm, a function on the imputed data. 
  This is useful, for example, to compute a cubic version
  of a variable, a transformation like Q = W/H^2 based on two variables, 
  or a mean variable like (x_1+x_2+x_3)/3. The so derived variables might be
  used in other places in the imputation model.
  The function allows to dynamically derive virtually any function 
  of the imputed data at virtually any time.mice