Howto create FAT32 bootable USB device with gpart?

[Hartmann, O.]

  Trying to create a bootable USB device (memory stick) with gpart - and 
failed. I need those USB mem sticks for BIOS flashing purposes on an 
older main PCB, utilizing FreeDOS. The old BIOS is capable of booting 
off from USB mem sticks, since I booted and installed FreeBSD via this 
method. But the creation of a bootable FreeDOS USB mem stick failed.
I have FreeBSD 9.0 as a mastering operating system and 8 GB sticks. 
These are the boundary conditions. Is there any way to perform this task 
with gpart?

Thanks in advance.

Oliver