Yeah, I was confused when you said the 175 wasn't intended for a spoolgun when Miller specifically offers the option.
I guess either find someone who has the <$100 adapter to either buy or see so you could cobble your own, find a used spoolgun with the Miller adapter, or upgrade.
I'm not sure why you say it wasn't intended for a spoolgun. Here is Miller's chart:
From here: Spool Gun Hookup Chart | MillerWelds - https://www.millerwelds.com/support/system-setup/spool-gun-hookup-chart
You just need the adapter mentioned.