Where x1 to z2 are the bounding corners of a rectangular prism region, similar to /fill.
structure is either a nether fortress, ocean monument, or witch hut (or any other structure capable of continuously spawning mobs without the use of mob spawners).
For example, /transform 0 0 0 100 100 100 Fortress will turn (0, 0, 0) to (100, 100, 100) into a region capable of naturally spawning wither skeletons, assuming all other spawning conditions are met, like how a naturally generated nether fortress would. (I think this only works in the nether, but whatever). I believe this can be done by editing the Fortress.dat file in the Minecraft world save folders, but whatever works. This should mean that the region should continue spawning wither skeletons naturally even after the mod is disabled.
Similarly, an overworld region transformed into a monument will spawn guardians naturally, while a region transformed into a witch hut will spawn witches naturally.