Lightly moisten the leader of an undeveloped roll of film (saliva works) and insert into the target cartridge an inch or so. Tighten the spool on the target film, pull out the moistened leader. With luck or practice the two leaders will be gently and quite temporarily joined and the target canister will give up its leader. I've done it, so I know it works, but it is a little wiggy. (Or you can just get a film retriever somewhere).