Okay time for my rational answer
There is no guarantee of this working, but here's my idea here:
go into the common events menu, make a common event, make a key input processing event (Or the equivalent, just use the event button that allows you to assign a key to a variable) that activates when you press whatever key (You'll need to assign it to one of those variable whatsits), then make a note of a number - When you see the list of keys, there should be a number next to all of them
I.E. shift( 8 )
remember that number well. (Not eight of course, unless that is the actual number you selected ;o;)
Then make a conditional branch. Remember that variable whatsit you assigned the key to? make the conditional branch activate when that variable is equal to that other number you memorized.
now this is where it gets sort of sketchy, all that stuff you did just now was the part where you press the button, this next part is where you make that button do something. The rest of this is theory, and i have no idea whether this part works.
Go in the conditional branch, this is where the rest of the events go. Select "move event" (Or the equivalent.) Make sure you're moving the hero and than input "increase hero move speed"(Or the equivalent.)
That's my idea, and i'm just going to throw it out there. Of course, you're going to want it to turn off or else this is just going to make your hero faster and faster. there isn't much of a difference, it'd just be something like:
key input processing
conditional branch if variable = whatever then
conditional branch if switch is on then
decrease hero move speed
switch: whatever off
if not
increase hero move speed
switch: whatever on
branch end
branch end.
Again, this is all theory, i use 2k3 but i've seen XP and it should work okay.