If you are interested in working on the ControlBlock sources, a place to add that functionality would be https://github.com/petrockblog/ControlBlockService2/blob/master/src/controlblock/SNESGamepad.cpp. You could take a look at https://github.com/petrockblog/ControlBlockService2/blob/master/src/controlblock/MAMEGamepad.cpp, where a simulated keyboard is already implemented.
The ControlBlock emulates two individual gamepads on your system. If you connect any additional USB controller it will simply show as another controller that is attached to your system.
So, there is no problem with having arcade controls together with any additional USB controller.
Contributions to the project are always appreciated, so if you would like to support us with a donation you can do so here.