This is just a guess for how to create a hooked rim from a smooth rim. Unlace the rim. Use a TIG welder to weld an appropriately sized aluminum rod of the correct alloy to the inside of the rim, thus creating a hook. This newly created hook will need to be milled and finished to allow the tire to seat correctly. The rim will most likely need to be heat treated to correct whatever happened to the tempering as a result of the welding. The rim will also probably be wavy and wobbly from the welding. I don't know how to correct this, other than to buy a replacement hooked rim. Re-lace the replacement rim.