Thanks for the hint! Winimage worked fine with the DOS 2.00 images, and I could copy the files into the image.
PC-DOS 1.x images can't be read by Winimage, though. DOS source code (https://github.com/Microsoft/MS-DOS/blob/master/v2.0/source/FORMAT.txt
) gives a hint as to why this could be the case: "The third to the last switch, "O", causes FORMAT to produce an IBM Personal Computer DOS version 1.X compatible disk. Normally FORMAT causes a 0 byte to be placed in the first byte of each directory entry instead of the 0E5 Hex free entry designator."
I don't need to manipulate PC-DOS 1 images, though.