Use Thorgrim's Map Editor, it delivers.
1. Add them to parties.txt first, map editor will load those automatically
2. By hands applying terrain up/down tool. Or delete the patch altogether and replace with copied plains from somewhere (see 3)
3. Either generate new map after setting new dimensions in map editor cfg file, or put copied patch of triangles to the edge of the map and weld vertices (CTRL+V) with those on the edge