Can’t say I’ve seen that method, they are normally just a very good fit, I’m presuming you can get the handles off again?
Just a thought, If I were doing it I think I would get the ferrules to be an almost too tight a fit then dry the handles out in a tent with a dehumidifier, fit all together then allow the ambient moisture in the air to tighten them up.
Ian
EDIT, just looked at your chisels again and I don’t think you need ferrules, the chisels have a half decent bolster end on them which should prevent the tang going in any further and splitting the handle.
Nicely shaped handles btw.