You could try using dd to put the image on the usb stick, but I don't honestly know if it would boot afterwards. However, what you could do is make sure you have a partition on the usb stick using the full space, hopefully about 4gb if it's a dvd, and then:
CODE
dd if=filename.iso of=/dev/sdb1
presuming that:
filename.iso = mandriva version you downloaded of the dvd iso.
/dev/sdb1 being the usb stick on your system - change this where necessary because I don't know what disks are in your system, etc. The command dd will read the dvd iso using the if parameter and then output it using the of parameter to the usb stick.
As I said, I don't know if it will work, but that would be how to write it to the usb stick from what I know.