Thanks for the help guys!
I was unable to find any resources on converting a 3di file to a known format.
As THQ Nordic has bought NovaLogic's IP rights, I sent them a request for releasing info about the proprietary 3di format. Right now I have had some contact with Reinhard Pollice, who seems to be the Studio Director of THQ Nordic.
On a sidenote: df4oed (developed by NovaLogic?) won't even let me view 3di files without crashing