Is it possible to accelerate the vehicle backwards in EDSMAC4?

No. In EDSMAC4, the inputs for the Driver Controls Throttle table only provide a forward acceleration to the vehicle. To accelerate backwards, as if the vehicle is in reverse and the driver steps on the accelerator pedal,you will have to use SIMON. SIMON utilizes a complete drivetrain model which allows you to shift the transmission into reverse gear and apply the throttle pedal, which in turn results in the acceleration of the vehicle in reverse.