I would not suggest using your normal maps as spec maps. Image example:
The bottom part is the red channel from a normal map, while the top part is a spec map. As you can see the normal map lacks a lot of detail. It's also too bright, even for the metal texture of this model. Putting that spec on a wood texture would be even worse :]
In case you would want to make your own spec maps I suggest you start with a "single color" (no details) value for green/red then load that into modelview and reload the model as you test new values. Just working with different tones until you find for example r/g/b = 80/80/80 on the red channel and r/g/b = 120/120/120 on the green channel might be a good specular base for that metal object you have made. Once you have "a base" which you like you can save that file as "a standard" for the next time you make something of a similiar material.
The next step would be copying details from normal/diffuse/displacement (if you have that) onto your base specular and gloss. You can also paint these details in.. For example edge scratches, dirt etc.
Also, DDO (
http://quixel.se/ddo/) is a great tool in case you just want to get some good values for specular/gloss maps. It's also a great for "a lazy texturer".. erhm that would be me. I do believe however that just by looking at the maps it creates one might get a better understanding of how to construct different texture maps.
Cheers & good luck!