I'd think it would be hard to add weight to a finished wood shaft except adding point weight or one of those screw-on adapters like 3 Rivers has.
If they are not flying right--that could be a spine issue: you can have light---heavy arrows still spine the same and fly great. I'd experiment with a couple differing spined arrows and see what tunes best to your bow: you can usually go up in spine a little if your bow is center-shot.
You could give John Christianer a call here in Conesville---he is a real pro a making wood arrows and has been around a long time: he could probably help you out:
TWIG ARCHERY 740-829-2847
Ben