how to PWM signal?

The FrSky TRF4-B receiver can output a pwm signal (by connecting channel 3 and 4). How can I make an Assan receiver (for example the 4 channel indoor receiver) do the same thing? Then it would be possible to use my Assan equipped Robbe/Futaba FC28 transmitter to function as a trainer transmitter with my new Futaba 14SG.

friendly greetings,