Can you do that? I don't think so, as it doesn't work after a quick try. Maybe there's a way around it? I'll experiment some more in the meantime.
I tried an alpine shader and there's a bug (?). Try this; add alpine shader to shader input of heightfield generator (I made the terrain 1000x1000m), seed 13536, hit generate... huge spikes around the center.
Got it. You'd have to add displacement to the image map. So I answered my own question, thanks again!
where did you plug in the image map?
In the shader input of the terrain generator.
Indeed, just plug the imagemap-shader as function-input into the displacement shader and crank up its displacement factor.
You can put an adjust colour in between to bias the greyscales of the imagemap to tweak steepness, for instance.