How can i add new picklist values to a custom objects picklist field(MetadaAPI)

import com.sforce.soap.metadata.*;

import com.sforce.soap.partner.PartnerConnection;

import com.sforce.ws.ConnectionException;

import com.sforce.ws.ConnectorConfig;

public class SfdcUpdateMetadata 

{

  static final String USERNAME = "username";

  static final String PASSWORD = "password";

  static PartnerConnection connection;

  public MetadataConnection metadataConnection;

  public void updateCustomObject(String MyCustomObject) {

 try {

   CustomObject co = new CustomObject();

   String name = MyCustomObject;

   co.setFullName(name + "__c");

   co.setDeploymentStatus(DeploymentStatus.Deployed);

   co.setDescription("Created by the Metadata API");

   co.setEnableActivities(true);

   co.setLabel(name + " Object");

   co.setPluralLabel(co.getLabel() + "s");

   co.setSharingModel(SharingModel.ReadWrite);

   Picklist expenseStatus = new Picklist();

   PicklistValue newpicklistval = new PicklistValue();

   newpicklistval.setFullName("NewPickListValue");

   expenseStatus.setPicklistValues(new PicklistValue[]{newpicklistval});

   CustomField expenseStatusField = new CustomField();

   expenseStatusField.setFullName("WSCCustomObject__c.ExpenseStatus__c");

   //expenseStatusField.setLabel("Expense Report Status");

   expenseStatusField.setType(FieldType.Picklist);

   expenseStatusField.setPicklist(expenseStatus);

   co.setNameField(expenseStatusField);

   UpdateMetadata updateMetadata = new UpdateMetadata();

   updateMetadata.setMetadata(co);

   //updateMetadata.setCurrentName("TheCurrentName");

   AsyncResult[] ars = metadataConnection.update(new UpdateMetadata[] { updateMetadata });

   AsyncResult asyncResult = ars[0];

   // set initial wait time to one second in milliseconds

   long waitTimeMilliSecs = 1000;

   while (!asyncResult.isDone()) {

     Thread.sleep(waitTimeMilliSecs);

     // double the wait time for the next iteration

     waitTimeMilliSecs *= 2;

     asyncResult = metadataConnection.checkStatus(

       new String[] {asyncResult.getId()})[0];

     System.out.println("Status is: " + asyncResult.getState());

   }

   if (asyncResult.getState() != AsyncRequestState.Completed) {

     System.out.println(asyncResult.getStatusCode() + " msg: " +

         asyncResult.getMessage());

   }

 } catch (InterruptedException ie) {

   ie.printStackTrace();

 } catch (ConnectionException ce) {

   ce.printStackTrace();

 }

}

  public static void main(String[] args) throws ConnectionException 

  {

    ConnectorConfig partnerConfig = new ConnectorConfig();

    ConnectorConfig metadataConfig = new ConnectorConfig();

    SfdcUpdateMetadata sum=new SfdcUpdateMetadata();

    partnerConfig.setUsername(USERNAME);

    partnerConfig.setPassword(PASSWORD);

    //partnerConfig.setTraceMessage(true);

    @SuppressWarnings("unused")

    PartnerConnection partnerConnection = com.sforce.soap.partner.Connector.newConnection(partnerConfig);

    // show the partner's session id into the metadata configuration then connect

    metadataConfig.setSessionId(partnerConnection.getSessionHeader().getSessionId());

    sum.metadataConnection = com.sforce.soap.metadata.Connector.newConnection(metadataConfig);

    sum.updateCustomObject("WSCCustomObject");

   //-----------------------------------------------------------------------------------------------------

   //------------------------------------------------------------------------------------------------------

  }

}

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