This has been discussed on another forum. Reportedly one maker is marketing "conversion" barrels for this swap. I for one would not trust this as the difference between the .45 ACP base diameter and the 10mm base diameter is MUCH larger than the difference between the .40 and 9mm. .40 to 9 with the same extractor? Sure. Heck the .40 shell holder on must reloaders is adequate for 9mm (adequate, not ideal). Try using a .45 shell holder to size 10mm? You will get a bunch of ripped off rims.
If you want to shoot 10mm out of a G21, buy a different top end.