I don't know Java, so this could be completely wrong. Have you opened the files in a text editor (Notepad++, VSC, or even Notepad) and had a look to see if there's something that resembles spatial data?

If so you can read the files in FME with the Text File reader and begin to filter out unneeded data and rebuild the geoms.

Thank you hkingsbury ! I'll give it a try.

I would expect the .class files to contain Java code as plain text, and not any data. The .jar files are basically renamed zip files and you can unzip them to see what's inside, perhaps there are some more common data files within, e.g. shapefiles.