You could use a ball end mill to either drill a new hole in your mold if there's enough space or you could run it down an existing hole to make the bottom round. Any machinist should have what you need and could modify the mold for you. (if we're talking about aluminum molds) You can pick up an non-USA made for around $6 if you wanted to modify it yourself.
Sorry for the big pic, only one I could find that clearly shows what I'm talking about.
