That’s odd that you couldn’t create a post! There should be a large red + button at the lower-right of your screen for creating a new post. I’ve split and moved your question to the correct location.
This should be fairly straightforward to do. There is a mask input at the bottom of most filter devices; you can use this to indicate to WM which areas should be affected and which left alone:
You can either manually draw a mask using the Layout Generator/Shapes device, programmatically create a mask from height/slope/etc, or import it from a file if you already have a map indicating where your buildings and roads are.
You can also go to “Open Examples” and open tutorial 9; you’ll see this exact process being used to erode the terrain around a road without affecting the road.