Replace 2 AA batteries with 5Vdc phone charger
Yup. Put a resistor in series with the LEDs to limit the current a bit. Start with 22 ohms. If the total LED draw is 100 mA, the resistor will drop 2.2 volts, giving about 2.8 volts to the LEDs. Use a 1/4 watt resistor or better. (0.1 x 0.1 x 22 = .22 watts, and this will make the resistor too hot to touch.) You can then experiment with lower resistor values, but don't get carried away. Depending on how the LEDs are connected, you might run into what's called "firecracker mode" failure. That is, with too much current one of the diodes fails. This increases the voltage across the others, causing the next-weakest to fail, and so on in a frenzied accelerando of failure. Just a caution.