That's slighltly easy to do

Just add this code snippet in a blank above main:
CODE
class Game_Character
def speed(value)
@move_speed = [value,0].max
end
end
and then, via move event, use the call script function and call it this way:
CODE
speed([number])
where
[number] should be replaced with the desired speed (let's say... 1.2; 3.5; 4.4... or whatever).
Negative numbers are not allowed.
I hope this can help,
Jens