code: public void concatField(String targetField, String concatString, String... sourceFields) {
          TransformationStep step = new TransformationStep();
          step.setTargetField(targetField);
          step.setSourceFields(sourceFields);
          step.setOperationParameter(TransformationConstants.CONCAT_PARAM, concatString);
          step.setOperationName("concat");
          steps.add(step);
      }
docs: Adds a concat transformation step to the transformation description. The values of the source fields are
  concatenated to the target field with the concat string between the source fields values. All fields need to be
  of the type String.