Terrain edit problem

I am new on unity, so my question might be stupid, I made a scene on Maya, and I want to edit the terrain on unity, like place grasses and trees, however, I can only do this on unity when I create a square terrain, where I can see Terrain script and edit. so I wondering, how can I edit my maya made terrain which is not square?

you could try the Object2Terrain from the wiki site.