If you import PSD file it should be prepared in special form. First, press Export button to see how the PSD is organised, there are layers responsible for color, depth, metal, specular, etc.
- They consist of four painting layers:
So each image can paint multiple channels at the same time.
To add normal map info you need to edit Height/Depth layer. The “zero” height of this Layer corresponds to “middle gray” or (127, 127, 127). Darker values will create valleys and lighter values will create peaks.
Hope this help