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I imagine that the spektrum transmitter also speaks PPM so it would likely be fine.

I've just completed a proof of concept of this today for my own project. I used the XJT external transmitter module and the XM+ receiver, both from frsky. I'm not sure if you can do it directly through the x9d but I was able to control the XJT external module (you can find on amazon). I supplied 9v ...