Change them into Walkers, Then when they leave the trigger, go back.

force_zombie_walk()
{
		self.lastRunAnim = self.run_combatanim;
		var = randomintrange(1, 8);         
		self set_run_anim( "walk" + var );                         
		self.run_combatanim = level.scr_anim[self.animname]["walk" + var];        
}

set_old_zombie_speed()
{
		self set_run_anim( self.lastRunAnim );                         
		self.run_combatanim = self.lastRunAnim;
}

You can change there speed in your "_zombiemode_spawner.gsc". Copy it from your root/raw/maps folder to your maps folder in your mod ( mods/yourmapname/maps ).

Open it and search for:

set_run_speed()
{
	rand = randomintrange( level.zombie_move_speed, level.zombie_move_speed + 35 ); 
 
//	self thread print_run_speed( rand );
	if( rand <= 35 )
	{
		self.zombie_move_speed = "walk"; 
	}
	else if( rand <= 70 )
	{
		self.zombie_move_speed = "run"; 
	}
	else
	{	
		self.zombie_move_speed = "sprint"; 
	}
}

Then you can change there speed in this function. You can use "walk", "run" and "sprint".
Hope it helps you. 

you cant just change the values in the set_run_speed method in _zombiemode_spawner.gsc. that sets the speed for the zombies when they first spawn and then is never used again in the spawner code.
you need to do something like what i said to do and call a function that will set them to walkers and save their last speed and  then call a method that will set it back to their original speed