Very interested in knowing what kind of modules and/or devices we can connect to use as additional outputs. Being able to use Expert sleepers modules would be awesome.
synthor (the main interface you see) supports usb midi hosting, and the ssp acting as usb audio device (not host).
it does not support hosting additional usb audio interfaces.
generally the underlying OS is linux, and so supports usb sticks, ethernet and usb audio interfaces - but you will need to configure these yourself. (there are various forum topics on this)