The usbserial module is included with most Linux distributions. It is not usually loaded automatically, because it is only useful for testing. It only supports one vendor/product pair, so that its new_id facility is not implemented.
It's quite possible that you will come across a variant of a standard device which is not supported by default. Many times a manufacturer will take a standard serial USB chip, embed it into their device, and give it a different product number. Look at the output of lsusb, and if it identifies the serial device, try installing the appropriate driver using modprobe. Then tell it about the new number using "echo VENDOR PRODUCT >/sys/bus/usb-serial/drivers/DRIVER"
You can find driver names by looking in /lib/modules/`uname -r`/kernel/drivers/usb/serial/.