The Contour Grid Tool can be used to add extra cells to existing grids. This method will only add cells to the bottom line of the original grid. You cannot add cells to the top or sides of existing grids.   

1. Make sure the relevant grid layer is active and select the Contour Grid Tool from the Tools Palette. The bottom right corner of the grid will become active.
2. Press B on the keyboard to undo the last point which completes the last cell in the grid.
   
   
3. Double click on this last point and you will be able to start a new row at the bottom left of the grid by clicking new points, as before. From here, the grid can be continued as normal.