How can I apply a "levels" operation on a single layer's roughness channel only?

[yoohasz]

Hi! I have already painted a layer with some noise on the roughness channel. I would like to tone it down, but keep the exact same noise pattern. I would need something similar to what blender's map range node can do: "bring down" the highlights, and "bring up" the darks of the roughness channel of my specific layer. How can I do that relatively non-destructively? Or at all? Thank you for helping!