you can event that yourself, here you can read a bit about it:
*klick*not sure about the battle rate though, that might require a little script call(one or two lines)
I'll see if it's possible with eventing, if not, I'll provide you with the script call
Edit:
Insert this inside the Scripts, below Materials but above Main:
# With this you can set the Encounter Rate of the current Map using
# $game_map.set_encounter_step(x)
# with x being the value you'd set inside the Map Properties
class Game_Map
def set_encounter_step(steps)
@map.encounter_step = steps
end
end
You can change the encounter rate by using
$game_map.set_encounter_step(x)
x being the desired value