My understanding is no, but if you just wanted a display then you could draw polygons for each square.
If it is acceptable for your squares to be defined in terms of longitude and latitude, then our Grid Imp application could probably be used:
http://www.winwaed.com/products/mappoint/gridimp.shtml
You talk of "post code sectors": Are you in the UK? If so, are your required km squares GR 1km squares? Grid Imp doesn't support these at the moment, but it is a possible area of future extension.
RB