To a first approximation, it's the radius of curvature which determines the strength of the lens, not its thickness - and Poser won't render a particularly accurate simulation anyway. If you do want to make a variable focal length, simply scaling the lens thickness would work fine since that will also increase the curvature / reduce the effective radius.

A lens with one convex and one concave face is called a meniscus lens - and it can still magnify or reduce depending on the relative radii. Contact lenses always have the inner face concave so as to fit the curvature of the cornea. Guess who's father used to be an optician? :)