Which USB floppy drives can read HFS-formatted 400K & 800K floppies?
It looks like you are going to need to use an older Mac to read the disks, due to the complexities involved with the variable speed drives used to format those disk, unless you want to re-program your floppy drive. I am not sure if you can do it with the Iomega drive especially since its USB, you would probably have to take it apart to do the next option.

According to this forum there is a way to control a floppy drive to get it to read those 400/800K disks. So it looks like you need a internal floppy drive and KryoFlux a USB-based floppy controller. Note This is going to require some hacking fun to accomplish...

If that is not your thing, then you should find an old Mac with a floppy drive to read the files and move them to more modern media via networking / apple-share etc. I am not sure what model of macs with floppy drives could read and write 400K and 800K disks, so there is another question to ask, and here is the answer Floppy Drive Observations: A Compleat Guide to Mac Floppy Drives and Disk Formats. So it looks like any Mac with a SuperDrive 1.44 MB type floppy drive will work at reading those older 400K and 800K diskettes. 

FYI, a really good source on this information relating to the Mac 400/800K drives. Working with Macintosh Floppy Disks in the New Millennium