Any question answered...

Or retap it to Italian thread?

It's not a retap, the diameter is substantially different and so is the pitch. To convert French to Italian, you first have to bore out the shell to 35mm (and I mean bore, its more material than you'd want to remove with a reamer), then cut new 36mm Italian threads. I'd be surprised if a bike shop had the necessary equipment, but the first part shouldn't be too hard for any machine shop with a modestly sized manual vertical mill of the Bridgeport pattern. Success is contingent on having enough wall thickness in the shell to leave it in a safe condition after increasing the bore by 1mm. Once you're past that, it is as others have said a simple matter of adding a 1mm spacer each side and Robert is your father's brother.