Often, the old specular map is a good starting point to make the roughness map. An easy way to do it is by inverting the image it in your preferred graphic editor, and then reducing the contrast until the higher and lower values match your desired threshold. Old specular maps usually have extreme 100% whites and blacks which are unrealistic for PBR

In my experience, it's advisable to save the new map as grayscale in sGray profile, and set its gamma correction to 1 inside Poser, so the value you see in the map is the value you get in the render