It is not possible to manipulate an axis of an existing joystick device(by VoiceAttack or any other application), however you can create a virtual joystick with vJoy and set the axes using
this plugin.
If the game offers a way to set this value using the keyboard, that would be a simpler solution. The original IL2(1946 and its predecessors) allowed you to set the throttle to an absolute value in 10% increments using the number keys, but it appears BoS does not(or at least it's not mapped by default according to the keymaps I can find).
You could try to use the throttle +/- keybinds to set the throttle to either extreme(minimum or maximum) so you know which value you're starting with, then increasing or decreasing to reach the desired setting(E.G. reduce throttle to 0 by pressing throttle- 10 times(or 11, if BoS still allows 110% power), then press throttle+ 5 times for 50%), assuming +/- still change the value in steps of 10 like it used to.
Whether such an option will work does also depend on the response of the sim to such a rapid change; If it has too much input lag, or the engine too little inertia, cutting or firewalling the throttle like that could damage the engine, or the change in torque could affect the attitude of the plane.