'APPLYING' Normal Map inPhoto/Gimp

I have an already rendered image done in Blender. No normal map. I made the normal map in Photoshop using a plugin. Can i actually ‘apply’ it in photoshop or Gimp somehow. I’ve searched, found nothing, im thinking there might a good reason for this. As normal maps need to ‘rendered’ with the normal map. Everything i have found talks about ‘making’ the normal maps, but nothing on actually applying it.
I just want my rendered image to have the appropriate normal map applied after the render. Thats all. Is this possible? A dumb question? Im not sure. Any help appreciated.

to use a normal map in blender internal it is set as a second image texture. The UV coords must be set and selected in the material properties, also normal map selected in the texture properties, and nor selected in the material part. Then render the image. If you are tryiong something totally different I don’t quite understand.