This procedure was designed for casual users, which usually own a single workstation. But it can be simplified a bit for large installations.
You can do this:
1) Install FP5 in any workstation.
2) Make a copy of all Forest files which are installed in the Max folder. You can find the list here
(search for the <maxdir> section).
3) Install FP6 (that will uninstall FP5).
4) Copy to the desired Max version, the files from step 2.
That should result in a functional installation of FP5, but using the assets of FP6.
This is not a big problem, since most of assets are compatible (perhaps with the exception of the new libraries included in FP6).
In case you miss some library (i.e. HQ Plants free which was removed in FP6), you can make a copy from "C:\Program Files (x86)\Itoo Software\Forest Pack Pro" before of step 3.
As you can see, the key are the files stored in the Max folder, which can be deployed by a script once you have them.
I hope that helps.