Wow that is a challenge sense most of the cooking done with cannabis contains fat (THC binds well to fat). Hmmm the only thing i would guess is to "fuse" the cannabis with the jello mixture. Add an extra 30-60 minutes to the boiling time of the jello mixture (as indicated in the box) but at the same time reduce 25-50 degrees. Once its simmered for the 30 minutes to an hour, strain using cheese cloth, add your rum and let it cool. Even then it seems unlikely. But yeah its the only way i can possibly imagine you would be able to bind the THC in cannabis with the jello + rum mixture.
Hope it helped a little bit, but its unlikely to work (at least i think it is).