No because that is the smallest unit in WC3 terrain.
Disable the fine grid. That grid is smaller than the smallest terrain unit. Even pathing units are 2*2 on that grid and they are smaller than terrain units.
Use SC2 which has a much finer terrain grid. WC3 terrain grid is exactly the number of units you specify in map dimensions. A 256*256 map will have exactly 257 discrete terrain nodes in each direction for you to modify.
The reason it only allows you to modify the medium grid intersections is because those are the actual terrain nodes. The texture around them is dependant on the data of nearby terrain nodes. This is done to allow blending such as is seen by the snow in the image.