Was that on the G3 iMac, under Mac OS 9.2 or 10.4?
Is StuffIt Expander installed and active? If so, you could check whether the StuffIt Expander preferences are set to mount images (choose not to instead).
Anyway, download the Network Access Disk 7.5 file (preferably when the machine is running Mac OS 9.2). Use StuffIt Expander to decode the MacBinary (.bin). The self-expanding archive (.sea) will probably open automatically (otherwise, double-click on the .sea file). Locate a Network Access.image file inside the resulting Network Access folder.
Now, try Disk Copy 4.2 under Mac OS 9.2. Do not double-click on the image file. Instead, launch the Disk Copy (4.2) program. Choose to Load Image File, and select and open the Network Access.image. Then, click on the Make A Copy button. It may or may not be possible to create a floppy this way through a USB drive.
If Disk Copy 4.2 does not work, try Disk Copy 6.3.3 (Mount Image under the Image menu) with the Network Access.image file. The mounted image will be called Network Access. Insert a floppy in the USB drive before you choose Make a Floppy from the Utilities menu in Disk Copy 6.3.3.
If the above does not work, again mount the Network Access.image file using Disk Copy 6.3.3. Open the mounted image (Network Access). Inside is a System Folder. Open this System Folder and remove all files from the three folders Apple Menu Items, Control Panels and Extensions (throw them into the Wastebasket/Trash, do not forget to empty the Wastebasket/Trash). The size of the System Folder is now somewhat smaller. Try to drag-copy the System Folder onto an empty (freshly Mac-formatted) 1.44 MB floppy disk in the USB drive. This may or may not work.
Otherwise, you may want to test the Mac OS X dd procedure (the LowEndMac article).
Alternatively, you may want to use an old Windows PC with a built-in floppy drive (and Aladdin/StuffIt Expander and WinImage).
Jan