set onClickListener for spinner item?

Hook to OnItemClickListener of Spinner. Then check whether the selected item is "Add new category".

If yes, show the dialog to add the new item.

While adding the new item,

1. Remove the last item "Add new category".
2. Add the new category entered.
3. Then Add the item "Add new category" again.

This will make the "Add new category" item as last one.

Code Sample :

layout main.xml :

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="fill_parent"
android:layout_height="fill_parent"
android:orientation="vertical"
android:weightSum="10" >

<Spinner
    android:id="@+id/cmbNames"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content" />

</LinearLayout>

layout spinner_item.xml

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

<TextView
    android:id="@+id/tvName"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content" />

</LinearLayout>

Activity class :

public class MainActivity extends Activity {

private static final String NAME = "name";
private static final String ADD_NEW_ITEM = "Add New Item";

private SimpleAdapter adapter;
private Spinner cmbNames;
private List<HashMap<String, String>> lstNames;
private int counter;

private OnItemSelectedListener itemSelectedListener = new OnItemSelectedListener() {

    @Override
    public void onItemSelected(AdapterView<?> arg0, View arg1, int arg2,
            long arg3) {
        HashMap<String, String> map = lstNames.get(arg2);
        String name = map.get(NAME);
        if (name.equalsIgnoreCase(ADD_NEW_ITEM)) {
            lstNames.remove(map);
            counter++;
            addNewName(String.valueOf(counter));
            addNewName(ADD_NEW_ITEM);
            adapter.notifyDataSetChanged();
        }
    }

    @Override
    public void onNothingSelected(AdapterView<?> arg0) {
        // TODO Auto-generated method stub

    }
};

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.main);

    populateList();

    cmbNames = (Spinner) findViewById(R.id.cmbNames);
    adapter = new SimpleAdapter(this, lstNames, R.layout.spinner_item,
            new String[] { NAME }, new int[] { R.id.tvName });
    cmbNames.setAdapter(adapter);
    cmbNames.setOnItemSelectedListener(itemSelectedListener);
}

private void populateList() {
    lstNames = new ArrayList<HashMap<String, String>>();

    addNewName("abc");
    addNewName("pqr");
    addNewName("xyz");
    addNewName(ADD_NEW_ITEM);
}

private void addNewName(String name) {
    HashMap<String, String> map = new HashMap<String, String>();
    map.put(NAME, name);
    lstNames.add(map);
}

}

You SHOULD NOT call OnItemClickListener on a spinner. A Spinner does not support item click events. Calling this method will raise an Exception. Check this.

You can apply OnItemSelectedListener instead.

Edit :

spinner.setOnItemSelectedListener(new OnItemSelectedListener() 
{
    public void onItemSelected(AdapterView<?> parent, View view, int position, long id) 
    {
        String selectedItem = parent.getItemAtPosition(position).toString();
        if(selectedItem.equals("Add new category"))
        {
                // do your stuff
        }
    } // to close the onItemSelected
    public void onNothingSelected(AdapterView<?> parent) 
    {

    }           
});

As far as adding "Add new category" to the end of the list is concerned, I think you should better go for custom adapter in which after adding all your items, you can add that constant ("Add new category") to end of array so that it should come last always.