Trimming edges off terrain

Any functions/plugins that can resize a terrain without affecting the details placed on it? How does anyone build maps with Unity terrain without having huge bare areas left over on the edges? Or if they want to add a new area to a map without stretching out the hills etc, how do they do it without making more resource hog terrain objects? I want to use it for indoor stone hallways etc, but the areas beyond the walls should just be a solid black background.