Cameyo’s suggestion was to add the wrinkles after you have snapshot the model into its final position on the canvas. Then you can use that alpha as a stencil (import to Alpha palette, then click “Make Stencil” – see the Stencil chapter of the manual for more info). Paint through the stencil to add your wrinkles to the scene.
Using the bump viwer material, you can paint details on using TextureMaster. What you do is load that material and use it as the active material for the model that you are going to texture. Texture your mesh using greyscale values. You will see the bump applied, rather than seeing the greyscale texture that generates that bump. When you finish creating the map, export the model with it still applied as a texture so that you can receive the benefit of ZBrush’s export routines. The map will be saved as a BMP of the same name as the model.