The trouble with specularity maps is that they are more attuned to the material of your object that you are representing than they are to the diffuse map representation of the object. You can use a graphics editor to tweak a diffuse texture map to represent your required specularity settings, but it takes time and practice to accomplish this in a realistic fashion and isn't always easy with maps with multiple types of spec.

Converting Diffuse texture maps to specularity maps is always going to be a bit more hands on, this can be made a bit easier with specific software such as Quixel which has a Texture map baker that allows you to tweak on the fly in real time on your actual model selecting various base materials which will be represented in your specularity map.

An example of where conversion of diffuse maps becomes an issue is something like a Snooker 8 Ball. The texture map has two sections, the majority black of the ball and the smaller white area with the number 8 in it.

The issue is that the specularity map for the ball needs to be just plain white as the entire ball has an even specularity over its entire surface, which represents the material the ball is made from not the diffuse appearance. Converting the diffuse texture map will give you a black section with 0 specularity and a white section with %100 specularity and no amount of tweaking will give you the results you require(Except for filling the entire section white). In this case and many others the diffuse is totally irrelevant to the specularity of the object so it can be hard to build from.

Here is a promo video using Quixel to give an idea of how this kind of thing can help in your workflow.