Unfortuantely, unless you have access to the source code to the Brook UFB to reflash it, you’re not going to be able to easily use the switches on the Supergun.
The UFB’s code expects the “hold turbo, press button, release turbo” to enable turbo functionality. It wouldn’t know what to do with a switch.
In theory, if you got super-creative, you could always design an additional PCB for your pushbuttons and switches, and this PCB connects to the UFB, to give yourself an “artificial” turbo functionality. Meaning that this PCB would take your button presses, and if the turbo switch was set to “on” for that button, it would send a “turbo-fied” signal to the UFB.
I’m oversimplifying the solution, but there’s probably a whole lot that can go weird here in practice. But conceptually, that’s the idea…
Or design a different PCB that can convert the slider switch to “When slider for Button-X is set to on, hold turbo, press Button-X, release Turbo. When slider for Button-X is set to off, hold turbo, press Button-X, release Turbo.”, and then wire up this PCB to the UFB’s turbo and button signals.