Repackaging .pkg files without installing/unpacking
You can use the Flat Package Editor from Apple.


Go to Apple's Developer downloads site and download the Auxiliary Tools for Xcode - Late July 2012.
Once downloaded, open the DMG.
Find PackageMaker.app and open the package contents, then navigate to Contents/Resources.
Copy Flat Package Editor.app to /Applications or elsewhere then open it.


From this app you can open a .pkg and edit the contents by dragging files out, editing them and dragging them back in, then saving the package.



Packages can be resigned using the button on the title bar: