PS3 battery uses 3.7 V or 5.0 V, depending on model. Both are considerably higher than 3.0 V from 2 x AA battery.
And how are you going to charge piggybacked battery? Most lithium battery are charged using a method called delta-V. By piggybacking battery (assuming they are the same voltage), you increase the capacity. Delta-V becomes harder and harder to measure as charging capacity increases. Each charger are tuned to charge specific capacity.
If a charger cannot measure delta-V correctly, it'll overcharge the battery. With lithium battery, you are looking at exploding battery.