You can get a metal lathe and then find the right size tubes and cut your whistles then add tuning slides. but I cant say I recommend this haha. This line isn’t a serious suggestion.
I’d just sell them and try to buy tunable ones. Selling used will have a loss, you could then try to buy used to not lose as much.
You may be able to find a maker who will do it. But labor is expensive, and theres still risk involved which could destroy the instrument. Its hard to just stick a finished whistle on a lathe, it wont be balanced right. Its possible to cut it and then chuck the mouthpiece so that its just a balanced tube spinning. but theres still a risk in damaging the mouthpiece. Would work better on the lathes that the mouthpiece could go through the chuck and be inside the lathe, so only the tube gets chucked. But just the chuck could leave marks on the tube. So I expect getting anyone to make them tunable will have a pretty steep price tag, at that point it may be better to just get tunable ones.
I KNEW North Stoke was in Mali, but nobody believed me.
His Kerrywhistle website has this email: kerrywhistles@btinternet.com
apparently in the UK, which is in North Mali, the music district
It is August, the UK is getting heat waves, some people may be taking
holidays around now, I’d be patient.