Adding trees dynamically

You can indeed, but it's through the $$anonymous$$ghly undocumented and unsupported TerrainData class

You'll want to play around with terrain.terrainData and inside that, treeInstances.

treeInstances is an array of TreeInstance objects, each of w$$anonymous$$ch you can set position on

Your best bet is most likely to just start out by cloning a TreeInstance, and changing the position (TreeInstance is a struct, so you can easily clone by assigning another TreeInstance to a new variable)