The MIDI input jack on our devices follows the MIDI Association’s TRS MIDI spec and uses an optocoupler for proper isolation and to help prevent ground loop issues. Because of that hardware design, it can’t be changed in firmware to also act as an external footswitch input.
Some older devices from other companies used their TRS jacks in less standardized ways, often to support things like external tap tempo or switch control. But as TRS MIDI has become more standardized and more commonly used for external control and clock sync, combo implementations like that have become less common.
There is a discussion on this thread about various ways to get certain sustain pedals to work with the cv/exp input or create your own custom footswitch box.