It is not recommended to connect a 120V only generator system to a 120/240V transfer switch because you will be feeding 120V to only one side of the switch hence not providing power to 1/2 of the switch.
There are 120V only type transfer switches that will work with the packaged EU2000i and EU2000i companion that we offer.
30A Manual Transfer Switches
Just select the 120V only versions.