That wouldn't be possible
The way that retarded ignition works is by pumping fuel into the cylinder even when no power is required, however the fuel isn't ignited by the spark plug. The fuel/air mixture is simply pumped out of the cylinder exhaust valve, where it ignites and expands in the exhaust manifold. This creates the flow of gasses when the driver is off of the throttle.
The engine isn't pumping gas, it is pumping fuel that combusts to create the gas : describing the engine as a gas pump is a gross oversimplification.
KERS does not increase the fuel flow through the cylinder relative to throttle position, as it provides power electrically. If a driver is off throttle (running at the FIA specified 10% limit) activating Kers will not increase the amount of fuel delivered to the engine. Thus, it cannot "pump" gasses out of the engine because it doesn't provide any additional fuel to create more expanding gas.
Retarded ignition allows fuel to be burned without delivering additional power : Kers allows additional power to be generated without burning extra fuel ...
One cannot be used in place of the other; they serve completely different purposes.