Analog input is not(yet?) supported by VoiceAttack, so you'd have to do it via plugin.
It depends on how the game you're using it with is designed, but usually whichever input method is used last will set the value. The problem with analog inputs is jitter; Because their value is usually changing constantly, they're always the last used input.
You would need an input filter, which would eliminate jitter for you, thus allowing the keyboard commands to work, or a way to not send input to the game when you don't want to use the joystick.
You could have the input from your analog throttle sent to a plugin, which would only send it to the game when VoiceAttack sets a variable.
Either way, no easy way of accomplishing such a thing natively at present.