I'm guessing but I expect you'd need to change the heater box, and that is somewhere you seriously do not want to go.
Both doors off, disable airbags, steering wheel off, all the front dash & associated trim out & the main internal crossmember, plus probably a few things I've forgotten. Also the actuating mechanism (most of which is buried deep inside the heater box and not readily accessible) on climate models is constructed using rather flimsy plastic gears & linkages. There have been several reports here of linkage failures; individual parts are not available from Fiat & the complete heater box is around £1000; you could likely add the same again for labour if you let Fiat anywhere near the job.
Sorry to be so negative but I'd say this particular modification just isn't worth the candle.
If buying a used model with climate, ensuring the unit works properly is an essential pre-purchase check.