IMO the quickest way to migrate from another platform (and to be sure its right) is to deploy with clonedeploy, clonezilla, or what ever you captured the image with to a disposable system and then turn right around and recapture with FOG. Don’t let the target computer boot into the target OS, but just deploy and then recapture. That way you can also use FOG’s more advanced compression tool zstd over gzip which clonedeploy appears to use. If you deploy to a modern computer or a vm on a fast server it will take you less time then extracting, converting, trying to understand the FOG meta data and then recompressing the files. Remember you are dealing with GBs size of files here.