If this is the terrain in the advanced lighting package, it is built from an rgba height map located textures/terrain/terrain.dds
You can try editing this, though I've only gotten mixed results with this and Evolved hasn't answered my questions regarding this file.
Alternatively, you can manipulate the vertices after it is built with code probably with native vertexdata commands and it looks like AL has some terrain painting functionality in include/terrain.dba, include/terrain extra.dba though I haven't really worked with these and i don't know how complete they are as the whole thing is still somewhat WIP