After two weeks of restless work I could manage to develop Mac OS X device driver and firmware loader for vendor specific usb device (Qualcomm Gobi Mobile Broadband device).
qcserial USB driver for linux directed me to the right direction. First of all, I was trying to upload firmware to the USB device after making some changes to SimpleUSBNotification example shipped with XCode. I had to port the codes of for Mac OS X.
Though I could upload the firmware after few days of work the device were not working as I expected. Because, the device and all its interfaces (class, subclass, protocol) were defined as vendor-specific. As a result none of the existing usb modem driver were recognizing it.
Then I was planning to develop a driver for this. My target was to expose a vendor-specific USB Interface (Interface #2) of the Composite USB device to be exposed as a port under /dev/tty.* using the driver and then use that port to connect dial-up internet.
Then I found a usb modem driver called AirPrime. I have been trying to modify this driver so that the modified source codes work for my usb device. And finally the success came to me. I could load the driver during boot which do recognize my usb device after uploading the firmware and then I can connect internet without any problem
Please let me know if you want me to upload the modified source codes. Thanks for your time.