Your Hobart is a constant voltage power source for the GMAW/FCAW process (wire welding)
it will have 120 AC output for the hobart feeder that it was intended for. That said you can hybrid your Miller 54 to the hobart without much problems, works this way, you pull your gun trigger that controls/energizes the wire drive motor as well as the shelding gas solenoid and a control signal is sent to the power source to close the weld contactor. Your going to need a wiring diagram from both to come up with the hybird control to correctly interface the two. You may want to pick up a old Hobart model 27-4300 feeder that will plug right in, there pretty cheap. Your Miller 54 could have a hubble twist lock or a Amprinol plug, Hubble is old world Miller, Amprinol is the later. You could pick up a older Miller single phase C.V power source and if in the last 20 years will be a direct plug in with the Amprinol early will be hubble. My opinion is your opening up a can of worms trying to change a 3 Phase Hobart to single.