If you have not found it already I think what you are asking is how to divide your model. If this is the case load up your model and under the geometry tab select divide. I usually divide a model around 6 times sometimes 7 depending on the size of the case mesh. If you had a base of say 1000 polys dividing this 6 times would give you a mesh of 4096000 polys to sculpt on. You can now begin to sculpt in all your details to the model.

Hope this helps,

Leon