It sounds like they work just like LOR boxes in DMX mode. I send one universe of data to my LOR boxes. A universe is basically 512 DMX channels. You've been confusing me cause you keep saying you can't send more than one channel to the fixture but yet the fixture is actually working with 9 DMX channels. I'm assuming you mean you can't send multiple DMX addresses. That's the same as with LOR boxes. They don't know what universe they reside they just receive 512 channels of DMX data and they pickoff whatever you've programmed them to do.
You really haven't told us what actual hardware is sending the DMX data. What are you connecting to the PC? What are you configuring in the xLights setup? Is it going to a controller card or a USB dongle? We normally send E1.31 to controller boards that have DMX outputs and then configure those boards to route a particular universe to the DMX output. Like I may send 12 universes to a Falcon board and then I tell the Falcon board I want it to send universe 7 out it's first DMX output. All the DMX devices on that output chain have no clue which universe they are they just know they are getting 512 channels of DMX data. LOR box #1 knows to pull off the first 16 channels and box #2 pulls off channels 17 to 32.
You can treat your DMX fixture chain as a string of RGB nodes. Each group of 3 nodes would control each fixture but you would need to use the ON effect to manipulate the red, green, and blue to control the 3 channels for each node. Node #2 controls DMX channels 4,5,6 and node #3 controls DMX channels 7,8,9.
I wanted to create an effect that was specialized for DMX control but so far there doesn't seem to be consistency in how DMX channels are used in all the available fixtures for me to be able to create a nice common control. I have a DMX controlled light and plan to play with it some day to work on a scheme.