Yes, you can trim the leader and insert the first couple of centimetres onto the spiral in the light while the rest of the roll is still in the canister; this is much much easier if you set your camera not to rewind the film all the way back into the canister but to leave the leader showing. Don't pull out more than the 8cm or so that gets exposed when you load the roll into a camera. Once you have the spiral primed, put the whole lot into a dark bag and wind away until you reach the end then just snip the film off at the canister with scissors and put the spiral into the tank.