Is it possible to add a another layer of clear coat, or two or three, after a car has been painted and the first layer of clear coat added? Would this offer better protection against scratches....and would this dull the color of the car?
Yes you can re-clear a car to add durability. With many paints it'll give it a deeper look once cut and buffed.
Wet sand w/ 400 and 600 grit (may be able to get away with only 600 to keep the MIL thickness up)reprep, mask and spray away. Personally I might mix it a bit "hotter" so it bites into the existing clear better...kind of like a blending mix of clearcoat.
2005 Ford F150 SuperCab FX4
1964 Chevrolet Impala SS
1998 CSVT: 354HP/328TQ @ 10 psi, now gone