One thing you can do is put the handles in a couple hundred dergee oven for a half hour, then gently crush where the ferrule sits with a hammer.
After you install the ferrule, dip the handle in water for a second.
You can probably pare and use the chisel lightly, but I wouldn’t pry a mortice or anything without the ferrule. It partly depends on how long the tang is and how it fits in its hole. The greatest risk is seating the tang without the ferrule to prevent splitting.