You can sand down a 22.2mm ISO alloy stem to fit a 22.0mm French headset. Use a rough grit paper to start, work to fine grit wet/dry. Wrap the paper around the stem and use a twisting, rotating motion - rather than sliding the paper up and down the stem. It will be round enough. This isn't a rotating axle we're talking about, and the actual "holding" part is being done by the expander wedge and slanted stem end, the combination of which are very much not round. Anyway, I've done it and it works fine.
This is easier than the alternative, which is to hog out a French stem's 25.0mm clamp to accept a 25.4mm ISO handlebar. I've done that with a drill press and it is a pain in the butt.