I don't understand why you want the weight, cost, and maintenance complexity of a full suspension bike for a city commuter. If you are concerned about comfort suspension seatposts and suspension stems are widely available and will do the job just fine on all but the roughest pavement. And those parts are much less expensive and require far less maintenance than a full suspension bike.
Also, if you retrofit a suspension fork on a non-suspension bike you will change the geometry of the bike, which will have a negative effect on the handling of the bike and probably on ride comfort as well.
Another thing to think about is due to the flexing of the frame, a full-suspension bike frame will have a much shorter lifespan than a non-suspension frame.