Yeah, I'm going to go with "You can't" for this one - "Postdoc" is inherently post-PhD, and the expectations of the position will pretty much require one. For example, many require establishing yourself as an independent researcher - which means grant writing, which is next to impossible without a PhD.
As I have just posted in another thread, I did in fact get a position that was notionally a post-doc without having a PhD. In the end, I think it was due to my specific skill set being more useful to the project than someone with more research experience. I have tried to repeat the trick, but without luck, so far. As GP mentioned, the positions are very often tied to funding that requires a PhD be hired.
