In theory, you could get 9x12 film, find some old filmpack parts, and assemble them. However, I think you might find that lots of film gets destroyed in the process. Perhaps just some 9x12 film holders, and use your Bergheil one sheet of film at a time?
I also have a 9x12 Bergheil. When I got it there was a glass plate holder (I think) and an adapter to use 4x5 film. I don't know if the 4x5 film holder adapter was a common accessory, but you might try finding one of those, because it will fit modern 4x5 holders and film.
You can see many more images of my Bergheil at http://www.gordonmoat.com/Bergheil/ and see the adapter. I included the Shen-Hao as a comparison in size, because the Bergheil is really small compared to a modern 4x5.
Gordon Moat Photography