1. There's a node called "file input". You can import grayscale heightmaps, or rgb maps (by default, brightness value will be used as height). You can also set it to interpret as rgb, in which case color values will be preserved.
2. If you have the heightmap to that existing terrain, yes. World machine is very widely used in the game industry, due to the flexibility it offers when editing existing terrain data. You can also create your own features and add them to the existing heightmap in any way you want.
3. I seem to have already answered that in "2", but lets elaborate a bit. There is a concept called "masking", which is the core modelling tool you will use a lot at advanced stages. You can select an area, isolate it, add to it, add a specific effect only to it, whatever! Each device has a mask input, where you can input a black and white mask. The effect will only be applied to the white areas.
4. If you already know the fundamentals of manipulating digital color data, you are halfway done already. WM is a height map based terrain creation tool, you can use grayscale maps in infinite ways, including using it for terrain elevation.
5. What you are working on is a square, 32 bit raw image data, the moment you create something in wm. You can export this as a mesh, but essentially it's still just an image file, converted to mesh. There are a lot of output formats, I export mostly 16 bit float Tiff heightmaps, and 8 bit tiff colormaps. Use them anywhere, it's just a 2D color or grayscale image.
I hope that answers your questions. I'd recommend digging into the tools first, it's easier to answer questions that are specific and categorical. Also, do join the fb group, linked in your other post. Enjoy!
