I am assuming you would like to add a command to Standard Firmata for DMX. If that is true, I can try to point you to code that may help.
The first step is to try the example provided in the DMXSimple library or writing a simple sketch on your own using the library. If that works as expected you can then go on to Firmata.
PyMata, ( https://github.com/MrYsLab/PyMata
), has a sketch that adds support for the Tone library. The sketch is called NotSoStandardFirmata and is located in the ArduinoSketch folder of the project. It also contains a Python based Firmata client that has the accompanying driver to use the added command. If you look for the play_tone command in pymata.py (in the PyMata folder) you will see an example of how that is accomplished.