I don't think that there is anyone on this board who actually does kernel hacking.
Therefore, all we can give you is general advice: Use the source, Luke.
You might get somewhere by subscribing and posting a message to the
freebsd-hardware@freebsd.org mailing list.
As a suggestion, it might simply be a matter of adding an id to a usb ethernet driver. A lot of those USB modems use a usb ethernet device as their interface. Examining the usbdevs -v output might be the place to start.