A high wattage soldering iron is the best bet in an existing model, which I did to my first model, after realising the difficulty of getting grease or oil in to an already installed shaft. Having learned from that experience, I now build grease tubes in to all my prop shafts before installation. I usually make my own prop shafts, so adding a grease tube is no big drama, just an appropriately sized hole in the shaft tube, in the correct place of course, silver soldered in. I make a cap tube about 3cm long, of a size that is a neat slide fit over the grease tube, and seal it with a piece of brass sheet at one end. After filling the prop tube with grease, I use waterproof outboard motor grease, the prop shaft is inserted and connected up. When it is necessary to top up the grease, some is inserted in to the cap tube using a small hypodermic syringe obtained from our local vet, without a needle, then the cap is slid over the grease tube, forcing the grease in to the prop tube. I have been asked if grease, rather than oil, places an additional load on the motor. It may do, but as none of my boats are high speed jobs, and all are driven through a belt and pulley reduction, no adverse effects have been noted, and grease is easier to prevent from leakage than oil.
Peter.