As far as I know - there is no way to edit the terrain of the neighbourhood except prior to adding it to the game. This one method requires Sims City 4 and is not a precise science to say the least. The controls for shaping the terrain in Sims City 4 are just not that precise.
Alternately you can modify the max slope value - see the post by
boatnana at MTS2 here for more information on this. This will not necessarily change the way things happen in placement - but may offer you more options for placement after the game does freaky things to the terrain.