There's no "existing" scripts unless you're taking them from other game.

Speaking of 1.6 version, you can't control doors in there. For this case you should get CE version. It contains a "loop script" function that will check if all conditions met.
Let's say, we will have "map.enemy" variable. We need to add it in FSM file of our enemy with INCREMENT action, right in the DEATH state. So once that enemy is dead, it will count +1 to our variable. And, we need to kill 5 enemies to proceed next.

In map script we need to put this:
door lock x y z //x y z should be changed to the door placement

And in loop script, we're putting this:
if $map.enemy == 5

{

door unlock x y z

}

Once again, all this will work in CE version. Get it here:
https://cg8516.itch.io/easyfpseditor-ce
And this is a link to more advanced online manual:
https://pixelwolf.net/efpse/wiki/index.php?title=Main_Page