How to add custom fields to item in Questionnaire resource

I am using HAPI FHIR models as dependency in my project.
How can override getItem() method in MyQuestionnaire class to return item[] which contains customField?

   // HAPI FHIR model class to represent Questionnaire Resource.
  public class Questionnaire {
    protected List<QuestionnaireItemComponent> item;
  }

  // HAPI FHIR model class to represent single item in Questionnaire Resource.
  public class QuestionnaireItemComponent {
    protected StringType linkId;
    protected UriType definition;
  }

  // this class is to add additional fields to Questionnaire resource
  public class MyQuestionnaire extends Questionnaire {

    // this throws compilation error.
    @Override
    public List<MyQuestionnaireItem> getItem() {
      if (this.item == null) this.item = new ArrayList<MyQuestionnaireItem>();
      return this.item;
    }
  }

  //  this class is to add additional fields to item.
  public class MyQuestionnaireItem extends QuestionnaireItemComponent {
    protected StringType customField;
  }

This was already asked on chat.fhir.org here - https://chat.fhir.org/#narrow/stream/179255-questionnaire/topic/Questionnaire

Please allow time for a response in one place before posting a question somewhere else.