DDI is a sector-based disk image format created with the DiskDupe program. The files are probably not the same as the DDI files commonly found and used in some MSX emulators.
There is a Pascal program floating around the internet which includes source code and can convert DDI files to raw sector dumps. It is called DOSIMG and can be found mainly on various Chinese websites. However, the program works only on DOS formatted disk images, as it relies on reading the BPB from the boot sector.