The best way would be to combine both your ideas. Cut the shaft in the middle. Make a fibreglass or carbon hollow extension of the length you want plus six inches and of the same cross section as your shaft. Make it double thickness. Then grind off three inches on either end down to the internal dimensions of your shaft. Insert and expoxy into place.
You could also make it an extendable paddle by buying a carbon ferrule with lock. Then you can play with the length until you find what you like.
JohnK

: Is it possible to lengthen a paddle? I have a carbon/fiberglass paddle that
: came with a whitewater kayak I bought that's around 175-180 cm long. Its a
: really nice paddle and I already have a whitewater paddle I was thinking
: about trying to make it a little longer to around 195-200 cm so that I can
: use it as a regular paddle. I can only think of two ways I might be able
: to do this. One is to add a carbon ferrule(its a small shaft that would
: fit into a ferrule). Another way would be to cut the paddle in half, carve
: a piece of cedar the same size as the shaft with the ends being able to
: fit into each side of the shaft, and then covering it with a carbon fiber
: sleeve. Does either way sound like it would work and which one seems like
: it would weigh less?

: Thanks.