Copper jointing:
You should be able to purchase copper olive O-rings that will slip over the outside of existing copper tube if and when you use a copper tube cutter to make the end smooth of no burrs. This joint requires a ½-inch (15 mm) or a ¾-inch (20 mm) brass male thread to poly adaptor fitting and a copper gland nut (crox nut) that must be placed over the copper pipe before the copper olive O-ring is placed on the end. Then sit the brass male thread over the copper end and pulling it back a bit from bottoming out bring the olive up against the thread and do the nut up tightly with two spanners one holding against the leverage of the other. Do not be afraid to over tighten this joint. It can handle it being a metal-to-metal joint.
You do not need to use tape or hemp with this kind of joint. However if it leaks tease out a small thread of hemp and roll it up on top of your trouser leg under your palm with a bit of spit. Then place between the end of the leaking olive and the male thread of the brass adaptor fitting winding clockwise as you look at the end of the copper tube. And tighten the joint.
This method of using hemp or rolled up plumbers PTFE tape is the same method if you use a hacksaw cut of your copper and run a groove around the tube with a ball bearing crox tool instead of using the ready made copper or brass olive O-ring.