Can either Spectrum or FrSky or Turnigy based transmitters be hacked to talk to arduino based micro-controller?
Is there an easier way to connect to & use just a module for this purpose?
I would assume the answer is "yes" for the FrSky Taranis without modification, "yes" for the Turnigy 9X and 9XR radios with modification, and "no" for Spektrum radios. The Spektrum radios are a closed system and modifications are unsupported and highly discouraged.
The FrSky Taranis has a USB port that could be used for bi-directional communication Ardunio and there is a Smart Port connection in the battery compartment. If you just want to do what the Companion9X does, you do not even need to modify the Taranis firmware. Otherwise, you will need to reprogram the USB interface portion of the Taranis.
The Turnigy 9X and 9XR does not have any bi-directional interfaces available. It is theoretically possible to modify the radio to provide a suitable interface. If you only need a unidirectional interface, then you could use Trainer port but using the module for this purpose means you would not be able to have a transmitter to control your plane. In any case, no matter what, you need to change the firmware (which on the Turnigy 9X requires hardware modifications).
What is it that you want to do with an Arduino? I might be able to be more helpful with a little more details.