You can but you're going to have to fix any major light or dark areas and even out the skin areas with an image editor like Photoshop or GIMP.  For instance, I made a bump map out of a head texture but first I had to fix the eyebrows.  Since they were dark on the texture they were dark on the bump map and created deep grooves above the figure's eyes.  The texture was that of a man who had a fair amount of five o'clock shadow.  Each whisker made a tiny pit because it was dark.  In photoshop I changed the dark eyebrows to white and I isolated the beard area and inverted it so that the stubble was white dots instead of dark dots.  Oh!  The texture is the one you see in my avatar.

Now the eyebrows show as raised above the surface of the skin and the stubble doesn't look just painted on.  You can almost feel it in your imagination when you look at it.

I hate when vendors make a texture and state "Includes bump map!" and you look and see that it's just a greyscale picture of the texture.  It's a lot of work to make a real greyscale bump map because you have to analyze the texture and figure out what's raised and what's lowered and then alter the greyscale image accordingly.  But when it's done right, it looks great.