will my Flysky FS-TH9X fitted with a FrSky DJT Module, and FrSky D8R-II Plus RX work with the new Pixhawk controller or will I have to purchase some kind of convertor for it all to work??
TIA 
Yes. It will work. You don’t need a convertor. The D4RII receiver puts out PPM sum, I cannot remember if the D8RIIplus does though. though you can program that (re flash the FW) with a version that does support PPM sum.
cheers …