I doubt that you can jump to a specific function by entering the function number, because when you enter a numeric value on the number pad the keyboard should perceive it as a value being entered for whichever function is displayed-- for instance, if you're on the Dual Voice function then entering a number should be interpreted as the voice number you want to select as the Dual Voice.
As far as I know, the only way to jump to specific functions in the function menu on the PSR-E models is to either press, or press and hold, a button that goes to a specific function. For example, pressing the Dual button will toggle the Dual Voice on or off, whereas pressing and holding the Dual button will take you directly to the Dual Voice function so you can select the voice that you want to use as the Dual Voice. There are only a handful of functions that can be durectly accessed this way, but you could go to whichever one is closest to the function you want to change, then go forward or backward until you reach the desired function.