Edit Change Terrain in game

Is it possible to edit the type of terrain (Im guessing) in game  (as opposed to say cartographer)

Raise/Lower Land : changes elevation

Summer/Twilight : changed 'Enviromet)

But is it possible to change the terrain: (Land, Chasm, water, desert, etc)

I see reference to 'PlaceTerrain'  but the following does not seem to work:

<GameModifier>
<ModType>Map</ModType>
<Attribute>PlaceTerrain</Attribute>
<TerrainType>Land</TerrainType>
</GameModifier>

My two gials if possible would be:

'Reclaim' unusable lands:  Ideally set a tile to Grassland, Plains.  (removing any mountain, chasm, water, etc).

'Create river'  I would love to see a spell to bring forth a river in a (or more) tiles.

Thanks in advance for any responses, or even just taking the time to read.