http://www.rqriley.com/frp-foam.htm
This describes the procedures used for one-off fiberglass construction. It's a basic technique used for the home builder...but it should translate well to a small production shop. More sophisticated techniques could be used for larger runs.
Basically, he "creates a shape" in foam by hand (this is where the CNC machine comes into this discussion) and once the shape is created, he covers it in a fiberglass skin and finishes off the surface.
Many people use pink styrofoam insulation (sold in 4x8 foot sheets and easily glued together for thicker sections) for the foam core of a fiberglass project. After you have a "block" of foam of approximately the right size, it's a simple matter to carve it. You can do it with either rotary tools, or a hot wire cutter...information on both of these techniques is easily available on this site and elsewhere.
Search around on this site -- there is at least one person who built a big machine for cutting boat hulls out of foam blocks. It's in this section...just do a search.
I hope I've given you a few ideas...a big machine would be fun to build, one of these days.
-- Chuck Knight