Looking for a way to add files into "packages"

I am looking for a way to add a file into a "package" on an Install DVD converted to an image file.

I know that Pacifist allows me to see into the contents of packages. However, I am looking for something that would allow me to add something, so that it would be included every time I do an install.

I suppose that checksums and other similar issues might complicate my quest.

I see that Apple has PackageMaker 3.0: how do I become a developer and/or otherwise obtain a copy?

Any information would be helpful...