The executable being packed in an executable format means it has to be decompressed on each launch. If it doesn’t it means it’s not saving any space anyway.
I don’t know what packing you’re looking for, but Windows applications are typically installed with installers. An executable compressed executable goes against this; unless you want to pack installers.
Traditional file compression works well enough. People know to launch an msi or exe or read a README. Introducing non-standard tools is not necessarily a good idea, and certainly is not intuitive to users not already familiar with it.