If you tell me your OS and what kind of .pkg file this is (macOS installer vs PlayStation vs other), I can give an exact command sequence tailored to your case.
Double-click the .pkg file to "install" it; this usually just places the "Install macOS [Version]" app into your Applications Use Terminal Commands: how to convert pkg to iso
If you are specifically looking to convert PS3 PKG games to run as ISOs on custom firmware (CFW) or HEN, the process often requires "liberating" the files first. PSN Liberator If you tell me your OS and what kind of
Let’s walk through a real example using free tools on Windows. how to convert pkg to iso