I don’t know of an off the shelf solution. But if you’re willing to get your feet wet in the world of DIY open source electronics, it would probably be a straightforward project to make your own.
For hardware you would need a push button rotary encoder-
-and something like an Arduino Leonardo to interpret the knob and act as a human interface device.
You would need to write your own code for the Arduino to make it and the knob act as a USB-HID, but there are many great examples of how to do this online. You will also need to wade into XPlane, to find how to configure an HID to interact with the simulator in the ways that you want- again not hard, as there is a huge community of sim enthusiasts who do exactly this sort of thing.
I have not done this in XPlane, and have not used FTS much. But years ago I did it using MS FSX, and by reputation XPlane is every bit as user accessible. It takes a bit of research and effort (and threatens to become a hobby into itself), but if you have the inclination it very likely can be done.