There are voices who tell that it's impossible. But this is not true at all: the truth is just that there isn't a nsis automatic decompiler out there.
This is what Scott told me:
"You can't decompile an NSIS installer, at present, because the decompiler would have to understand all of the different types of binary headers from all of the different versions of NSIS, as well as know how to identify any of them that have had their source modified (since its open source), so, nobody has yet to write this psychic program."

Honestly I think he's right, nobody would write such a general program, but everyone who is interested in reversing a nsis package should examinate the sources and understand how the installer is done.

Well, the only thing that you should discover is the registry and/or config files modifications during the install process, because the files that are copied are clearly available to you after a complete installation, isn't it?

So here's what I'd do, if I were you: I would extract all files simply installing the program on my HD; then I'd take trace of all config changes made during installation, including new or modified registry keys; finally I would use myself the NSIS installer package to re-package everything