5.7x28mm has a tiny little rim: (7.80 mm (0.307 in). which is only slightly larger than 22 rimfires. You'll need to do alot more than just tweak the extractor.
The .22 Reed Express firearm cartridge was designed by Reed's Ammunition & Research. It is a 7.62 x 25 mm Necked Down to a .22 caliber bullet. This cartridge gives better ballistic performance than the FN Five-seveN's 5.7 x 28 mm with the same barrel length, although at a price of lower magazine capacity.