Currently there is no way to set the entity of which a spawner will spawn. Right now there is only a .setEntityType() method which is fine in most cases, but when you want to add effects / armor to mobs when they spawn, it is not really helpful.

If this can't be done, another solution would be a .getSpawner() method in the CreatureSpawnEvent, and then the developer could add a meta tag to the spawner containing what type of armor the mob should have etc.