Actually, there is a way to, in effect, permanently add a state to an enemy via events.
It's not quite as kludgy as some of my no-scripting-skill solutions, either-- basically, when they learn how to fight a given kind of enemy, you turn a Switch ON-- know how to fight Fire enemies? Fire Switch is ON.
Now, we create a state called "Wimpy!" It has no animation, it doesn't go away, and it, say, cuts the enemy's stats in half or something.
Create a common event called "Wimpy Fire Enemies?" This common event will check if the Fire Switch is ON, and if it is, it inflicts the "Wimpy!" state on all enemies.
Now go to each troop of Fire enemies, and look at the Battle Events down below the troop. Create a battle event on turn 0 that will run only once, and have it call the "Wimpy Fire Enemies?" common event.
Ta da!
Yeah, it'd be cooler if you could do it the other way, but this should work.