Good news, I solved my problem. I was able to apply the proxy settings in PCC and use the getvirtualbox script. I have tried that before, but obviously I made a mistake, probably with the "no proxy" settings.
So here is what I did:
1) create a new network profile in PCC and activate it
2) set proxy settings for this new profile
Tick "same proxy for https
FTP-Proxy: left blank
no Proxy: localhost,IP (note: no http:// and Port needed here)
-> so maybe that is what I did wrong in the first place.
I still would be interested whether or not it is possible to make an offline install similar to the lomanager.
So thanks for your input,