uhg!
could you post more information? like the model and number of your sensor? can you/ do you want to modify the sensor circuit? can you take photos of the instrument PCB?
if you don't want to touch the sensor, then, you need a USB Host to serial...
I suggest you to check the
Vinculum series from FTDI I have used the Vinculum I in a
Vdrive2, It's a somewhat generic host (mostly oriented to usb flash drives) but it can handle a CDC connection... I think you can use a Vdrive3 with the new Vinculum-II... these new line of Vinculum processors can be user-programmed, and it seems there is a an
application note about a sample program to transparently transfer the data from usart to the CDC... (exactly what you need, but take in consideration that you need a programmer for this vinculum chip)
I suggest you to get a Vdrive3 and manage the CDC data by commands (or get also a V-prog and change the firmware to the transparent cdc-uart firmware)
Finally If your sensor uses a proprietary software in the PC you need to figure out the communication protocol by yourself before connecting to the Vinculum...