You can use a capacitor but the led will be dimmed til off, and due to the size of capacitor it could be inconvenient, I don't know about the internal space of DeWalts:

The tool should already have the LED resistor, so you just need to buy the capacitor and diode.
-Diode can be 1N4001-1N4007, 1N4001 works better.
-A 3300uF capacitor can hold the LED for about 2.5s before start to dimming:
led current = 23mA
input voltage = 18V
white led voltage drop: 3.6V
C = A * s / V
C = 0.023A * 2.5s / (18-3.6V) ~= 3993uF
-A 6.3V capacitor should be enough, 4V works too and are smaller.
There's another way to hold a led with a capacitor without dimming, but it will require more internal space. Let me know if you need it.