Increasing the size of terrain without destroying it.

So say I have a terrain that’s 1000x1000 meters with mountains and such. I want to add 100 meters on the top and right sides of the terrain making the terrain 1100x1100, but I don’t want any of the terrain already made in the 1000x1000 area to be affected.

How would I go about doing this? Would I have to export the heightmap into a raw file? If I did it that way how would I get the heightmap resolution to work correctly with it?

My desire is to be able to add onto my terrain when I wish. Does anyone have any idea how to do this?

Disclaimer: I am a complete noob in Unity and have not tested this suggestion.

I would try the following. First export the raw. Then open it in Photoshop, Gimp or an hightmap editor. Expand the image by 10% and fill/ paint the new area with desired hight /colour. Save as the edited image with a new name so we have a backup if it fails.
Back in unity extend your terrain by your desired 100 units and import the hightmap.

From my understanding of the systems that should work.