Long story short is that you can’t. Beatbuddy just doesn’t have that functionality.
However, since you have a Midi Maestro, you can control your volume with it.
Just like van9818 showed up above, use one of these CC’s depending on how you’re connected.
What I’m suggesting is that you set one button to toggle between 90% and 80%, and a second button to toggle between 70% and 60%.
You can also set these values to whatever you like, or even add another button if you wish, but this works pretty great.
You can go from high to low or vice versa within 2 button presses, which to me is even more efficient than an expression pedal (unless you only use your ear and do not try to hit a specific percentage).
I have the latest firm ware installed on my Mini Maestro and have attempted several times to get my expression pedal to use for volume control of the beat buddy. I go through the processes described on the web site, but it never takes. Sugestions?