var _doc = app.documents.firstItem(),  
    _pi = _doc.pages.firstItem().pageItems, 
    _pg = [], 
    _gb = [0, 0, _doc.documentPreferences.pageHeight, _doc.documentPreferences.pageWidth]; 
     
for (var i = 0; i <  _doc.layers.length; i++) 
    _pg[i] = _doc.pages.add(LocationOptions.AT_END); 
 
var bg_layer = (_doc.layers.itemByName('Background').isValid) ? _doc.layers.itemByName('Background') : _doc.layers.add({name:'Background', locked:false}); 
bg_layer.move(LocationOptions.AT_END); 
 
for (var x = 0; x < _doc.pages.firstItem().pageItems.length; x++) 
{ 
    var _dpi = _pi[x].duplicate( _pg[ _pi[x].itemLayer.index] ); 
    _dpi.itemLayer = bg_layer; 
    _dpi.geometricBounds = _gb; 
} 
 
bg_layer.locked = true;