Adding custom matchedData within .custom()

[ghost]

I would like to set other fields as matched data for retrieval later on when calling matchedData().
Is this possible?

e.g. (partial pseudo code)

body('user', ErrorCode.LIBRARY_USER_INVALID).isMongoId().custom((value, { req }) => {
  const user = mongoose.loadUserById(value);;

  // now set userType as a matched field so that this will be returned when calling matchedData(req)
  setAsMatchedData('userType',  user.type);
});

[gustavohenke]

I'll consider this as an equivalent of the rename feature, which some users have been asking about. #785