I unfortunately have not found the information to this for custom DTHs. As far as I can tell, GE has not published if they made some type of custom attribute for LED states. The ZigBee standard only allows the option of making the LEDs blink for a set period of time. If you want to change the settings you will need to do it manually as stated above.
I have written a custom DTH for ZigBee GE dimmers that at least simulates the fade to on and off (though GE doesn’t officially support that either even though the ZigBee standard can).
The DTH can be found here though since this is not officially supported by the GE switches, when turning off the dimmer level goes to 0%. That is a different behavior than other DTHs.