I don't think many people modify rated climbing equipment the way you are talking about, for various reasons. One comment that comes to mind is that the spike on the axe may very well be attached with glue as well as the rivet so removing it may not be as simple as you think....
Petzle makes an ice-axe with a trekking poll section in the bottom. You might look at that for ideas. Weather or not you need to have a machine-shop build you an insert is mostly a question of what skills and tools you have.
I can envision a plastic insert with a circular hole to take a tubing lower section. With a hole for a pin. Probably could be made with a drill press and some hand rasp work. Putting together a tubing section that would still be strong enough with the holes in it and not be really heavy sounds challenging.
You might consider building a trekking pole extention that straps onto the ice axe shaft without modifying the axe. I suspect it could be a lot less work to build. Something like the old ski-mountaineers taping there ice axe to there pole.