I managed to get a IDE/USB converter and just connect a DVDROM to the unit via the USB port. you just select USB from the boot list and off it goes...
you may be able to do it from a usb pen but it will need to be bootable and have the network drivers to access your share, getting dos drivers for the NIC may be tricky.
If you have an external hard drive you dont mind tweaking you may be able to connect a DVD drive to it, however, always a good idea to have something like the following handy;
http://www.ebuyer.com/product/130517