Here is the process that I am in the middle of to make ball end spokes. Let me start the conversation with the fact that I have not yet built up a wheel with these spokes to determine their integrity. I am making these from original 1890's spokes as I don't want to do a restoration to the bike and new spokes would not look correct. The spokes that I need are 308mm in length. Here is the process:
Cut the head off the "j" to make the blank about 310mm.
Turn/file/sand the butted section of the spoke down to the .063/.060 dimension of the center of the spoke (make sure it fits into the slot in the hub.
Thread the spoke with a 1-72 die about 4-6 threads (depends on how much taper or point is on the end of the spoke).
Thread on a 1-72 nut, adjust to the proper length spoke, in my case 308mm.
Silver solder the nut in place.
Chuck spoke and file the solder round.
Dip the ball end of the spoke into a product like Permatex Rust Treatment to dull down the cleaned metal from the turning.
I believe that these will work for a display bike, but the strength of the threads and the solder are yet to be proven. I may be wrong and the nuts will all strip off during the wheel build.