I'm not overly well versed in Poser, but this might work. Start with your basic tone (I don't know if this would be 50% gray or a fleshtone). Use the burn tool (set to midtones or shadows) and draw the depressed portions (cracks if you will) of the wrinkles. Now draw over those, in a slightly larger brush size, with the dodge tool, and you should get something nice and stripey. (the lighter parts should be right up against the dark parts, with midtones in the areas in between wrinkles) Clean it up and add some more texture with a small-sized smudge, and they should look like wrinkles. As for the Poser portions, I have no clue.