Excalibur can easily extend the length by your amount but extending the beginning is only one clip at a time. I have this one which will trim the front and back:

/**

 * This script will trim the front and back for all selected events.

 * 

 * Written By: Edward Troxel

 * Copyright 2004 - JETDV Scripts

 * Modified: 08-03-2005

 **/



import System;

import System.IO;

import System.Windows.Forms;

import Sony.Vegas;





var trimAmt = new Timecode("00:00:01:00");





try {



  //Go through the list of Tracks

  var trackEnum = new Enumerator(Vegas.Project.Tracks);

  while (!trackEnum.atEnd()) {

    var track : Track = Track(trackEnum.item());



    //Go through the list of Events

    var eventEnum = new Enumerator(track.Events);

    while (!eventEnum.atEnd()) {

      var evnt : TrackEvent = TrackEvent(eventEnum.item());



      if (evnt.Selected) {

        var dStart = evnt.Start + trimAmt;

        var dLength = evnt.Length;



	//Get current take offset

	var tke = evnt.ActiveTake;

	var tkeoffset = tke.Offset;

	tkeoffset = tkeoffset + trimAmt;

        evnt.Start = dStart;

	tke.Offset = tkeoffset;

    

        dLength = dLength - trimAmt - trimAmt;

        evnt.Length = new Timecode(dLength);    

      }

      eventEnum.moveNext();

    }

    trackEnum.moveNext();

  }





} catch (e) {

    MessageBox.Show(e);

}

Change the var trimAmt = new Timecode("00:00:01:00"); line to read:
var trimAmt = new Timecode("-00:00:00:15");

That will change it to 15 frames and by making it a negative time, it should extend instead of trim.