Here's a quick Python script that will export the DDP page extents. Just copy and paste into the Python window within the mxd. It will create a temporary layer called 'polys', which you could export to disk and use directly as the index layer.:
>>> polys = []
... mxd = arcpy.mapping.MapDocument("CURRENT")
... df = arcpy.mapping.ListDataFrames(mxd)[0]
... for pageNum in range(1, mxd.dataDrivenPages.pageCount + 1):
... mxd.dataDrivenPages.currentPageID = pageNum
... extent = df.extent
... polys.append(arcpy.Polygon(arcpy.Array([
... arcpy.Point(extent.XMax,extent.YMax),
... arcpy.Point(extent.XMax,extent.YMin),
... arcpy.Point(extent.XMin,extent.YMin),
... arcpy.Point(extent.XMin,extent.YMax),
... arcpy.Point(extent.XMax,extent.YMax)
... ])))
... arcpy.CopyFeatures_management(polys,r'in_memory\polys')
note: this will only work if the DDP pages are not rotated.