It could be quite difficult
Assuming you have an extra sheave you would first have to drop down a messenger and then fish it out at the exit hole. The problem is that there are other halyards in there, wires which run every which way for anchor and steaming lights, and possibly spreader lights, and then the spreaders or the spreader brace. On some boats there is an additional problem with the noise silencers which are usually mast shaped blocks of styrofoam that all the other stuff is threaded through which are pushed up into the mast at a couple of locations to stop the wires and halyards from rapping the inside of the mast and keeping everyone awake at night. The trick is to drop the messenger down without wrapping around or otherwise fouling the internal stuff. For example the main halyard normally runs behind the spreader brace and the jib halyards normally run in front of it. You would create a problem if your new halyard went around the main halyard and then went in front of the spreader. Internal halyards should be run when the mast is down. in some complex cases It would be necessary to remove all the halyards and wires and then run them all back up with all the stuff going in front of the spreader together in a jig so that none get twisted while threading them up. Then repeating the procedure with all the stuff that goes aft of the spreader.Note that I said COULD be quite difficult, It could also be easy depending on your mast. I would bet my money on difficult however, because Murphy wrote too many laws about how things can go wrong.