why do you think so?
If it is common pl2303 or ftdi based devices --- there are special modules which should be loaded before usbserial. If it is a completely generic solution, you need to load usbserial with parameters:
Aha, finally, it will be located not at /dev/tts..., but at /dev/usb/tts/0PHP Code:
insmod usbserial vendor=0x1234 product=0x5678