Not today*, at least not with the default RWG and XML edits that i know of. From what I understand, however there will be(MIGHT BE???) a slider in A21 that will allow you to have more control over how MUCH(ratio) of each biome, but won't allow for patches within, at least not in most random worlds. NOTE: cities CANNOT cross boundry biome borders at ALL in the RWG and there seems to be no way to prevent that. Biomes are placed first, then terrain(mountains, hills, rivers, etc), and THEN finally cities/towns, and then wilderness POI's, after all of that roads are drawn. That is why cities won't cross boundaries as the tile system won't let it, at least on A20.
* You can easily paint a map after you generated it the way you want the biomes, check the Mods section. You just open the biomes file in an image editor pick the right color, go nuts, and save it back.