well wood is both good and bad. the good news is you can splice an extension to the existing wood spar and then build up some more wing ribs recover it and make a longer wing, the bad news is the only way to test the spliced wood spar strength is to break it... this is not absolute necessary, but i'm paranoid. but basically what you do is cut the end of it at an angle of about 15 degrease (1 rise for every 15 across minimum) which makes for a long shallow cut across the spar cut a new spar piece to add onto it at the same angle glue add a lot of pressure and let it dry over a weekend. Also you need to install reinforcing plates.thin reinstall the new ribs to and spar of the new length. the best thing to do is to do test spars first. and then bend them till they break until you can achieve a standered bend. IE take a new regular spar clamp down one end and bend it till it breaks and mark that spot (usually on the floor) then cut a spar in half glue it back together after cleaning up the cut and see if you can get back to that point or exceed it.(once again not necessary but i wouldn't fly without doing this) on another note when you make the cut try not to sand it. I have done this on smaller planes with only 3" spars (models) and i make cuts using a hand wood saw and a miter box, and then use a razor blade to trim into it's final position. it is a long and argues process, but provides the best bond. try to avoid sanding as it leaves tiny particles inside the wood poors which weakens the joint. I have successfully on a model spar cut it and sanded it to the right dimensions and then used an exacto to scrape down to new wood, but would not trust that process on a plane i was going to fly personally unless i could reinforce it in some other way or start packing a chute...

alright i found what i was looking for had to dig it out of the back of the garage but i found my AC 43.13-1b/2a book (acceptable methods, techniques, and practices - aircraft inspections, repair & alterations) but in section 1-40 defines splicing of spars should be able to find this online, but if you need me to copy it into here let me know and just to warn you it was written by lawyers so it is hard to understand and very descriptive, but not in a helpful way.

I hope all of this helps and be sure to take pictures it has been a long time since i have worked on a "real" airplane so it's good to see i still remember something.