How do you make a .pkg?

Rip one apart and see what it is made of.

FYI: It is a tar file, I think passed through gzip.

tar -xzf file.pkg

would, I think, be the way to extract the contents of one. Double-check that though. Reverse the process for putting one back together.

You can find more infos about pkg at http://developer.cobalt.com/