Hi,
If you open the mapbasic window and enter the following commands:
Set CoordSys Table your_tablename
Update your_tablename
Set OBJ = MBR(CreateLine(CentroidX(OBJ) - 250, CentroidY(OBJ) - 250, CentroidX(OBJ) + 250, CentroidY(OBJ) + 250 ))
This will create a 500m x 500m grid - just change 250 to 500 for the 1km grid.
Take a copy of your table first just in case it's not what you want!!
HTH
Greg