I think you have to build a new head developed from a photo of the subject then exchange it for the Poser head. I've done this with various degrees of success with Lynda Carter and Princess Di. If you have a high-res close-up photo of Sigourney (needs to be exactly front view, no angles) plus an exact side-on view, I could give it a try. You could play around with morphs and expressions forever and never get close to your subject.