If you're not going to modify the original normal map, perhaps use a multiply node (Ctrl+M) and a Vector3 input (Ctrl+3) and hook them up. Try adjusting, for example, the blue channel by changing the Vector3 input to 1, 1, .5 or 1,1,1.5, etc. This is off-the-cuff, so pardon if any of the nomenclature I'm using is wrong. :)…