How to refill old Nike Air sole with CO2

Update 27Jul2017 The project worked. There was a substantial improvement in cushion. CO2 is a much larger molecule than N2 so it does not deflate easily. Meanwhile I tried inflating the other shoe with butane for even more cushion. The pressure was so high that the effect was a much harder shoe. As the shoe is heated by the foot, it eventually reaches a temperature around 35C and at this temperature the vapor pressure of butane is 3.6 bar giving you an effective 2.6 bar gauge pressure which shouldn't have been too much for my taste but i guess there was a lot of propane in my mix so the pressure must have been higher than. If this is not the case, a butane-pentane mix will be ideal. There are a couple ways to inject butane in the shoe and be able to seal it but they are rather messy and dangerous processes.