A machinist should be able to turn you a threaded piece of thick wall tube that screws in so you then can knock it out.
However, if you go for the cone bearing conversion, the bearing cups are of bigger inner diameter, so it will be impossible to ever get them out. I drilled 2 small 4 mm holes opposite sides from underneath in the headstock and used a 4 mm drift to knock the cups out. Once they where out, I threaded the holes to M5 and screwed a 5mm grub screw in with some Loctite to close 'em up.
If ever the cups have to come out again, it just needs to remove the grubscrews and screw in 2 M5 bolts to push them out