Some more information, for good measure
A normal map is a map that, oddly enough, calculate normals and is a fake sense of depth. It calculates how the light reacts to the surface, and at a certain angle it will be seen as real depth.
There are 3 ways Normal Maps are made.
1) Directly from GImp/Photoshop, or any other softwares. You make a height map, grayscaled, and use a plugin to calculate from that. This is used for less-important models for the normal, or for a normal_detail pass on detailed models. A very common thing to finish your work with!
2) From a High Poly Base model. This is how hard surface models are made, and usually a base for organic models. This will make a model look smother than it actually is, and usually gets stuff added from 1) right after it is made.
3) Sculpting software normal. Very simply a normal map, usually 90% finished, from a sculpting software like Zbrush. This only gets 1) used if the model is very complicated, and usually comes from 2).
To alter a normal map you got a few things you can do, without too much hazzle:
1. You can overlay new details over. By changing the blue (?) channel, ad overlay it, there will be no change in the originals detail. That way you can add new depth to a texture/odel.
2. You can change it. Usually this means using the clone tool and clone details, and copy/paste some parts. This can be very destructive!
3. Delete normals. By getting the right color, you can delete parts with easy and fairly non-destructive in photoshop.
So for your idea: Open both textures. Isolate the area you want, and get rid of the texture. Now get the other texture, copy it over and start cloning the details over, or copy/paste it. Then fix up any oddities, and save it. Fairly quick, but might need some tweaking to avoid getting weird streching or stuff like that.
Generally, it isn't too had to do. But you might need to clean up some, after altering a normal map. 1 change in pixel can mean a black spot on your texture. Nobody likes that
For the save. You want to save it as DXT5, for quality. The rest should be good on default. You might want to enable MIP maps if they are off. They help with performance in the length!
Cheers,
Matth