Type Command
Category Generation 
Command /#setbiome
Help Use /help #setbiome

Sets the biome of the selected area. You can use the Block Selector to mark an area, or use #selecting to run it from a command block.

This is especially useful for giving generating structures their own biomes for custom mob spawning. I recommend placing Generic Solid blocks below the structure where you want it to set the biome, adding a World Script Transformer to the structure that matches the block you used, and using it to run WorldScript:Command #selecting ~ ~ ~ ~ ~ ~ #setbiome plains on their positions. This will give you the most freedom with the shape of the biome.