It is possible to use a portable sustain pedal by connecting it to the external PEDAL jack on the rear panel of the HP-1500 and 1800.
It is not possible to use a portable sustain pedal with the HP-2800 or HP-3800 because neither of them has a 1/4 inch input for an external sustain pedal.
It is not possible to use a portable sustain pedal with the HP-2800 or HP-3800 because neither of them has a 1/4 inch input for an external sustain pedal.