a_event.target.content.onPress = function () {
    this._parent.startDragLoop();
    this.onRollOver();
};

(Main.as)

import mx.managers.DepthManager;
import flash.display.BitmapData;
//====================================================================//
class Main extends MovieClip{
    //----------------------------------------------------------------//
    function Main() {
        var self : MovieClip = this;
        trace("i Main");
    }
    //----------------------------------------------------------------//
    function onLoad() {
        var self : MovieClip = this;
        trace("i Main.onLoad");
        //----------------------------------
        // MyScrollPane
        var sp : MyScrollPane = self.createChildAtDepth("MyScrollPane", DepthManager.kTop, {
            scrollDrag : true, hScrollPolicy : "on", vScrollPolicy : "on"
        });
        sp.setStyle("borderStyle", "none");
        //completeイベント
        var spListener : Object = new Object();
        spListener.complete = function (a_event : Object) {
            trace("i ScrollPane.complete");
            //サムネイル作成
            …省略…
            …self.thumb_mc にサムネイル用MCを作成…
            …self.thumb_mc.frame_mc にサムネイルに表示する現在領域用MCを作成…
            …省略…
            //ドラッグ時の追加イベント
            a_event.target.addExtEvent({
                mouseDrag : function (a_event2 : Object) : Void{
                    trace("i mouseDrag");
                    //現在領域用MCを移動
                    self.thumb_mc.frame_mc._x = a_event2.target.hPosition;
                    self.thumb_mc.frame_mc._y = a_event2.target.vPosition;
                }
            });
        };
        sp.addEventListener("complete", spListener);
        //scrollイベント
        spListener.scroll = function (a_event : Object) {
            trace("i ScrollPane.scroll");
            //現在領域用MCを移動
            self.thumb_mc.frame_mc._x = a_event.target.hPosition;
            self.thumb_mc.frame_mc._y = a_event.target.vPosition;
        };
        sp.addEventListener("scroll", spListener);
        //画像を読み込む
        sp.contentPath = "test.jpg";
    }
}

(MyScrollPane.as)

import mx.containers.ScrollPane;
//====================================================================//
class MyScrollPane extends ScrollPane{
    private var m_extEvent : Object; //追加イベント用
    //----------------------------------------------------------------//
    function MyScrollPane() {
        trace("i MyScrollPane");
        //
        m_extEvent = new Object();
    }
    //----------------------------------------------------------------//
    function startDragLoop(Void) : Void{
        …省略…
        spContentHolder.onMouseMove = function () {
            …省略…
            //追加イベント mouseDrag を呼ぶ
            if (this._parent.m_extEvent.mouseDrag != undefined) {
                this._parent.m_extEvent.mouseDrag({
                    target : this._parent
                });
            }
        };
    }
    //----------------------------------------------------------------//
    // 追加イベント登録
    function addExtEvent(a_obj : Object) {
        for (var i in a_obj) {
            m_extEvent[i] = a_obj[i];
        }
    }
}