I am not getting you.

The script decompresses .PZZ files, and makes them .PZ3 files.
That script, right?

And you have an empty scene?
Like you have stored a .PZZ file, but with nothing in it.
Then you decompress it, put your stuff in it, then save it?

Maybe I am just not understanding what you are doing(or saying).

You keep saying that it is not a file.  Well, they are all files.  PZZ's, PZ3's, OBJ's they are all files.