Hi Irfan,
SITEs are usually only intended for standard cells and IO pads. To place blocks you can use Relative Floorplanning which includes an option to create an array of blocks. Here's a solution with examples:
http://support.cadence.com/wps/mypoc/cos?uri=deeplinkmin:ViewSolution;solutionNumber=11649020
Or Floorplan Synthesis which is a more automated flow:
http://support.cadence.com/wps/mypoc/cos?uri=deeplinkmin:DocumentViewer;src=pubs
;q=soceUG/soceUG11.2/masterplan.html#1036843
Hope this helps.
Brian