You can use the dd
Per the man page, you'll want to use /dev/fd0 as the "input file" (if) and the name of the backup file you want to create as the "output file" (of). Here's an example:
dd if=/dev/fd0 of=floppy.backup
This will create a copy of the disk in fd0 to your current directory as floppy.backup.
Reverse the if and of values to create a floppy from the image.