Some digital keyboards are digital organs with separate midi inputs to play pedal voices with a midi pedalboard.
If you wanted to add an organ pedal board to other digital keyboards, you typically had to use it to activate sounds in another sound engine such as a VST or midi "hardware" module. Yes, you can run the midi pedal board to the input of a keyboard, and if it could do a split keyboard with only the bass split responding to midi in, then this was feasible, but with inconvenience, and not all keyboards have that capability.
I thought I would mention the Hammond pedalboards: XPK-130G and XPK-200GL.
These are not the full-sized pedalboards a classical organist would need to play, for instance, the pedal part of a Bach organ fugue. But they are suitable for classical organ reportoire that just uses pedal points to add bass fabric to a piece (which covers alot of repertoire). They also are suitable for playing bass lines in a range of popular music styles.
What is interesting about these is that they have their own bass samples and rendering engine builtin-- they are standalone digital instruments. They come with 5 bass voices: Electric Organ, Pipe Organ, Synthesizer Bass, Fingered Bass, Upright Bass. You don't need a separate sound engine. With amplification, the smaller unit even could be used with an acoustic piano.
They are 24-bit samples. The Nyquist limit for the 33kHz sample rate (up to 16.5kHz frequency) is more than adequate to cover the bass pedal range.
I have not used or heard them.