You need to use photoshop or any photo-editing program for this, what i usually do when i create a spec map is i take my diffuse, make it black and white, then go to filter-other-highpass, then play around with levels and brightness and contrastHi, I am referring to do it in maya, but if you have a way to do in photoshop or some other program also will be welcome.
My native language is Portuguese.