Some people have done it, usually to get the Z wave mapping utilities that will work with the stick.
It’s important to understand that smartThings was not intended for secondary zwave controllers because they wanted it to always be a multi protocol platform, so they don’t include many of the utilities that a zwave only system would have, including controller shift.
For that reason, one of the issues that comes up is when you add new devices, they will not be visible to the secondary. If you want it to visible to the secondary, you have to remove the stick from the network and re-add it again. So basically each time it gets a snapshot of the devices that were available at the moment that it joined the network.
You can read more about the people who are using the sticks for mapping in the following:
FAQ: Is there a way to see the Zwave mesh network map?
All of that said, what’s your goal for adding a secondary?
Also, you probably already know the following, but just in case: the USB slots on the hub itself cannot be used by customers. So in order to add a USB zwave stick as a secondary, you have to have it plugged into a laptop or another processor which then runs the software which manages the stick. So the stick becomes another stand-alone device in your house, it’s not a physical add on to your hub.
Like I said, you probably already knew that, I just wanted to be sure. 