Hey @malcolm yeah you can, all you have to do is just add a paint layer on top of everything > set the roughness to passthrough > add a level to that layer and change the affected channel dropdown to roughness and edit that to you liking!
Check out the first icon in the layer's tab, the one that says "add effect". You can, at the very least toss in a HSL node in layer form, and make adjustments to the layer in question. They probably expect to add more controls (node like effects) here.