I'm not precisely clear about your objective - do you want to use the 'cells' of your custom grid as the the index layer for a map book? If you open up the dataframe properties you can save that grid as a graphic.. the issue is that it saves it on the layout side of the world, not the data side. You can select the graphic, copy it to the clip board, then flip back to the data side of things, paste that into your dataframe and then convert it to a feature class (that tool is under the drawing toolbar menu). It will convert as a grid of polylines that you'll then need to rescale, make sure is georeferenced correctly, and then convert to polygons, and then name each of your cells with what ever page number/name that you want to use in your map book.
That said, possibly it is easier to create a fishnet (in the Feature Class tools "Create Fishnet") - you'll need to tinker a bit to make sure the cell boundaries are where you want them but you can create a polygon feature class pretty easily this way.
