Using Web-XPIs for customized setup

I am trying to use some XPIs (Orbit 3+1 Theme, PrefBar, Aggreg8) with
the windows installer.
There is no problem installing these files within the browser, but I
would like to use them within a  customized setup.
I modified the config.ini to include these files, but whenever it tries
to install one of them it exits with an error.
Why are the setup XPIs different to the ones one can download
everywhere?
Is there a sane _and_ easy way I can convert these files, so that they
would work?

Thanks,
Victor