Hi,
You could search for the necessary signal-OUT needed for the interface or PC. How? With a free downloadable scope (check-out my website chapter 1) or eventually drawings of the MC15 (chapter 5) which has the same HF-module I think.
If you think you cannot handle this then you could by the original pupil-module (slave-modul) Graupner ref. 3290.3 from which you can take the signal (check my website chap 3 under Graupner connector)
For the most universal interface, you should make or buy a USB-interface (which transforms the TX-signal to joystick-signal). If it's for using with FMS (free), other (free) options are somethimes possible.
Check-out my standard mail here-under and study it eventually. Olivier