Re: how to modify the altitude of the specific terrain area?
Not a direct answer, just some ideas...
Within the osgEarth framework, the best thing to do is to affect the source heightfield before it becomes terrain. osgEarth composites multiple heightfield sources together into a continuous super-heightfield.
One idea would be a callback that let you have access to the osg::HeightField object after it was read but before it became part of the terrain. You could alter the values based on your deformation values.
Taking this further, perhaps you could even have a special type of heightfield source that applies a delta to heightfield values (instead of literal heights). You would use a source like this to deform any incoming heightfield data automatically.