Lengthening oars

I have two wooden oars which are about 1.8 m (6') long. They are too short to be usable for anything other than short day trips. The calculation in Roger Barnes' book The Dinghy Cruising Companion suggests they should be about 2.4m (8') on a Mirror. I was thinking of lengthening the shaft by using some sort of metal tube as a sleeve. Has anyone else lengthened theirs? How did you do it? It may ultimately be cheaper to buy new oars, but If I can use what I already have, I would prefer that. I am aware that 2.4 m may be too long to store easily on a Mirror, so I may end up compromising on the length.
Thank you!
Red Kite