A copy-paste solution from another post.
1.) Open Part 2 of BlizzABS. Locate 'def update_control' (Line 3592). A few lines below, find 'player.move_speed = player.normal_speed' (Line 3596) and replace it with
player.move_speed = player.normal_speed unless $game_switches[id]
Replace 'id' with whatever switch you want to be on in order to run your parallel process event.
Go down a bit and find 'player.move_speed = Config::RUN_SPEED' (Line 3605) and replace it with
player.move_speed = Config::RUN_SPEED unless $game_switches[id]
Again, replacing 'id' with the switch you want.
2.) Do something like this:
Drains MP as long as the run key is held down. If MP is zero, the character will walk at normal speed even if the run key is held down. If the run key is not being pressed, regen MP. The above example was just bare-bones testing.
EDIT: Forgot a couple of lines in the picture. Fix'd.
EDIT 2: Changed the eventing a bit to allow all actors to use this feature, not just the first actor.