I haven't seen this particular arrangement before. I do recall that there were threaded-to-threadless converter similar to the one you described but they usually had an internal clamping mechanism similar to a quill stem to hold them into the steerer tube, not the welding that you describe. I was hoping someone who knows more about Ibises (Ibi?) than I would answer this but I'll throw out four possible solutions for you.
1. All the steel Ibises I've seen have unicrown forks for which this solution will not work... but if yours has an actual fork crown you could get your local framebuilder to remove the old steering tube and braze in a new one.
2. Perhaps a local machine shop can drill out that welded-in tube and resurface the inside of your steerer tube to allow you to use a traditional quill stem.
3. Find a framebuilder to build a custom stem to your specifications to fit your current steerer tube extension. There are still a few framebuilders around who will tackle an unusual project like this. The one who comes immediately to mind is Paul Sadoff at Rock Lobster Cycles in Santa Cruz, CA.
4. Buy another fork. If your bike has 26" wheels you can probably find a nice beefy used rigid mountain bike fork with a 1" steerer tube. Make sure the steerer tube is long enough for your frame and that the fork rake is not dramatically different from your original fork.
Brent