You can generate normal from mask in PS and then combine it with car normal. Or model all the details on temporary mesh, properly bake to plane and combine using substance. Depends on what software you have and how much time you want to spend on it.
that method isn't mathematically correct btw, you need to actually transform the perturbing normal into the coordinate space of each pixel/base normal. then do the addition. * drag and drop normal map merger which uses the "correct" maths NormalMapMerger.zipUploaded Jan 11, 202369.66 KBnot full tested yet but I sure…