add-partial-diff-file.
The diff file was typically created with the save-diff-file command, or by a saved configuration. The start and size arguments specify the location within the image. If size is left out, the (virtual) size of the file is used.
If any unsaved changes are found in the image object, the command will fail. Such changes can be discarded by using the -force flag, or first saved using the save-diff-file command.