We would generally suggest connecting a Subwoofer to your connected amplifier, rather than the CXN itself.
However, if this isn't possible, you will be able to connect an active Subwoofer to the CXN despite there being no specific 'Sub Output' on the unit.
This can be done by connecting your Sub to the RCA or XLR outputs on the CXN.
Please ensure you have pre-amp mode enabled in the StreamMagic app device settings, which will allow you to control the volume via the CXN's volume controls.
We would also suggest starting with the volume on the Subwoofer and CXN set to a minimum, and then slowly increasing to begin setting up your system with the Sub.