Android : How to add dynamically several fragments

I create a little app with 3 static fragments and a view pager adapter. But what I want is to create Only one kind of fragment and with a button "add" , add other fragment which will be adapted by the view pager.. here is what i have done

FragmentTab1.java ; FragmentTab2 ; FragmentTab3 have the same code

import android.os.Bundle;
import android.support.v4.app.Fragment;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;

public class FragmentTab2 extends Fragment {

    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container,
            Bundle savedInstanceState) {
        // Get the view from fragmenttab2.xml
        View view = inflater.inflate(R.layout.fragmenttab2, container, false);
        return view;
    }

}

here is my MainActivity.java

import android.app.Fragment;
import android.os.Bundle;
import android.support.v4.app.FragmentActivity;
import android.support.v4.view.ViewPager;
import android.view.View;
import android.view.View.OnClickListener;
import android.widget.Button;

public class MainActivity extends FragmentActivity {

    //Button adfg = null;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        // Get the view from activity_main.xml
        setContentView(R.layout.activity_main);        
        // Locate the viewpager in activity_main.xml
        ViewPager viewPager = (ViewPager) findViewById(R.id.pager);

        // Set the ViewPagerAdapter into ViewPager
        viewPager.setAdapter(new ViewPagerAdapter(getSupportFragmentManager()));
    }   
}

ViewPagerAdapter.java

import java.util.ArrayList;
import java.util.List;

import android.content.Context;
import android.support.v4.app.Fragment;
import android.support.v4.app.FragmentManager;
import android.support.v4.app.FragmentPagerAdapter;

public class ViewPagerAdapter extends FragmentPagerAdapter {

    final int PAGE_COUNT = 3;
    // Tab Titles
    private String tabtitles[] = new String[] { "Tab1", "Tab2", "Tab3" };

    Context context;

    public ViewPagerAdapter(FragmentManager fm) {
        super(fm);
    }

    @Override
    public int getCount() {
        return PAGE_COUNT;
    }

    @Override
    public Fragment getItem(int position) {
        switch (position) {

            // Open FragmentTab1.java
        case 0:
            FragmentTab1 fragmenttab1 = new FragmentTab1();
            return fragmenttab1;


            // Open FragmentTab2.java
        case 1:
            FragmentTab2 fragmenttab2 = new FragmentTab2();
            return fragmenttab2;

            // Open FragmentTab3.java
        case 2:
            FragmentTab3 fragmenttab3 = new FragmentTab3();
            return fragmenttab3;
        }
        return null;
    }

    @Override
    public CharSequence getPageTitle(int position) {
        return tabtitles[position];
    }
}

activity_main.xml

<?xml version="1.0" encoding="utf-8"?>
<android.support.v4.view.ViewPager 
    xmlns:android="http://schemas.android.com/apk/res/android" 
    android:id="@+id/pager"
    android:layout_width="match_parent"
    android:layout_height="match_parent" >

    <android.support.v4.view.PagerTabStrip
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:layout_gravity="top"
        android:paddingBottom="10dp"
        android:paddingTop="10dp"
        android:textColor="#000000" />

</android.support.v4.view.ViewPager>

fragmenttab1.xml ; fragmenttab2.xml ; fragmenttab3.xml has the same code

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:orientation="vertical" >

    <TextView
        android:id="@+id/textView1"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="Fragment 1" />

    <Button
        android:id="@+id/bajou"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="Ajouter fragment" />

</LinearLayout>