I should mention that MCEdit is not a mod or a part of the game at all. It's an external program that edits save data. In that regard it is literally restricted to vanilla; the edits it makes has to coincide with what vanilla expects. Now, if you're intending to manipulate biomes on-demand in-game (such as for a minigame where the biomes change periodically during gameplay), then MCEdit won't work since that would be a one-time operation outside the game (so you may need to clarify that particular intent).
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