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.
    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;

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