Grout has gone hard and I have not finished can I add something to it to soften grout to finish the job
No.  Grout is basically a form of concrete.  If this technology existed then when the government needed to redo roads they would just add this magical potion and road would be redone.  A small batch of grout mixed properly only has a usable time of a few hours.  Once it even gets even a little stiff it is done.