So. I understand that you want to make an exception for an USB device.
Step 1. Create a plug and play general USB device definition.
Step 2. Block it with an device rule.
Step 3. After the blocking event reaches the DLP monitor go there and export the device parameters in a .csv file
Step 4. Go on DLP policy and create a plug and play definition importing the file that you've just created.
Step 5. Add this new created device as a exception on the device rule used on step 2.
Hope it helps,