Is it possible to switch guitar effects on and off within a preset? For example, with HIT button?
No, unfortunately not.
For this functionality, you would require a Switch-3 or Switch-6, whereby you could assign a particular guitar effect to a particular footswitch.
If you choose Guitar under Switch-6 Mode in the Setup menu, each of the four guitar effects as well as Guitar Boost and HIT are automatically assigned to one of the six footswitches for you.