I've added weight to a few of my shafts, one a 3/8"x10 and one a radial. On the 3/8x10 I drilled and tapped further into the shaft for a cut off piece of 3/8x10" joint protector screw I cut back. On another 3/8"x10 and on a radial I drilled and tapped further into the shaft for a piece of 5/16"x14 threaded stainless rod. In all three cases I added about .3 oz and cut a slot into the threaded insert for a screw driver to screw in.
The 5/16"x14 was the easiest and would be the way I would do it again, or if making a new shaft I would just drill and tap further.
The rod screws in pretty tight, but I also used some silicone sealant to lock it in but still get out if needed.
.3 oz doesn't seem like much, but if you mostly like the weight of your cue before, when you add that .3 oz in the front, you can also take off .2 to .3 oz from the butt and the adding and subtracting will make a noticeable change in balance.