But, it not as easy has just converting to caliber and rotor.
1. You need to change the Master Cyclinder
2. You need to change the proportioning valve
3. You need to replumb all the rear break lines
4. You need to rewire your emergence brake
5. You need to modify the rear hubs to except the caliber mounts
6. You need to rework the ABS system to except the new calibers.
So to answer the questions, yes you can, but the cost will greatly outway the benefit. May just be easier to buy a new car with 4 wheel disc brakes
But here a link to a kit that may make this worth your time.