Are you saying you have bass pedals-- that is, a pedalboard-- such as commonly used with organs?
If so, I think the answer to your question depends on how you've got the keyboard set up as far as how it uses the MIDI data coming from the pedals. Are you using the pedals to control the style, or are you using them to play on a channel that's separate from the style parts?
In general, the volume of a given MIDI channel is controlled by the Channel Volume message (CC #7), so if you've linked the MIDI IN channel that the pedals are received on to, say, one of the Song channels, you can adjust the volume of the pedals' notes from the keyboard by adjusting the volume of that Song channel. Or, if you've got the pedals going to, say, the Right2 voice, you can adjust the volume of the Right2 voice.
But if you're using the pedals to control the style, it might depend on whether you're using the "MIDI Pedal 1" or "MIDI Pedal 2" template, or a user-defined template. I'm not familiar with the differences between "MIDI Pedal 1" and "MIDI Pedal 2," but if the pedals are being used to play a specific style part then you would need to adjust the volume of that style part. If the pedals are being used to control the chords, I'm not sure the volume matters.