With a standard photo it will not be easy, the value of the pixel includes both diffuse and reflected light component. For non metals the spec colour matches the colour of the lighting so if that is known it may be possible to filter out the diffuse contribution. Another approach could be to use polarisation to remove scattered light when taking the photo. Also spec is very directional so unless even lighting was used to take the photograph you'll get a direction baked into the image that may clash with the scene lighting.

Depending on what you're using the spec for the faked bump map can be useful, for example a raised part of the map may be a different material or may be worn to have a smoother surface.