Please see http://www.win-vector.com/blog/2017/09/custom-level-coding-in-vtreat/ and http://www.win-vector.com/blog/2017/09/partial-pooling-for-lower-variance-variable-encoding/.
ppCoderN(v, vcol, y, w = NULL)
character variable name
character, independent or input variable
numeric, dependent or outcome variable to predict
row/example weights
scored training data column