I haven't done it myself, but I've heard of people replacing the plate in the holder with a black piece of cardboard and putting a sheet film on top of it.
By the way there is an adapter that is sometimes sold on eBay to use 6x9 MF rollfilm on Fotorok. I have it and tried it twice, but for some reason all shots were out of focus. I chalk it up to my inexperience focusing the Fotokor..
They may have adapted the back properly except to check the focal point on the ground glass. I have done 4x5 to 6x9 cm with success. You would be surprised how many cameras I found with incorrect focus either twin lens or slr. Even Hassleblad and bronica slrs. This is corrected usually by shims or rotating a viewing lense on Yashicamats Don