Blimey what do you want a hole that deep for ?.
Any way this is what you can do you need a couple of drills of the required size, with parallel shanks. You'll find the the shank ends are soft, set them up in the lathe and turn a spigot on the end of the shank then silver solder or braze fasten in some way an extension to this to get the length you want, you may have to do this on two or three drills making each one progresivley.
When you come to drill the hole first use an unmodified drill as deep as you can can get, then an extended drill as deep as that one will go and finally the longest modified one you've got to get the depth you want.
You'll need to keep withdrawing the drill to clear the swarf and give it a squirt of WD40.
You didn't say what machine you will do it on lathe drill or mill, you may find that feed travel is a problem you may have to keep moving your set up:- the tail stop forward to get feed thats why you'll need more than one extended drill.
I'd have a good look and a think at what your trying to do, a design change may get you around the problem,
allowing you to use a length of tube.
Hope this makes sense
good luck
Stew