Combining pre-existing heightmap with generated data to a single terrain?

I have a terrain I’d like to make for a game I’m making, made from 5x5 areas, where each area is 1km width & height.

However, some of these areas (specifically 6 areas out of the 25) are very specific and were created already, and I have their heightmaps.

I saw the file generator, but how can I use it on a specific location of a larger terrain?

I’m not positive if this is exactly what you’re looking for, but best I can tell, to get the results you’re looking for, you would just load the 6 heightmaps in on their own file input node, click the “set in layout” button, and align the tiles properly. If that’s not exact enough, you can manually place them at specific points in the dialog box, but in my quick experiment with it, it always wanted to change the size of the terrain, so make sure to make note of the size before adjusting placement to avoid that.

That being said, I would not be surprised if you got some pretty sharp seams where the heightmaps meet, so that may take some work to smooth out.