Well, here we have to question how we think of arrows. (Man, I've been sounding like a rules lawyer lately, and I promise you I'm not - I really don't care much, but I AM interested in what everyone thinks about it. Think of me more as a rules philosopher rather than lawyer).
So... the game makes no distinction between short bow and longbow arrows. There's just... arrows.
I haven't done much archery, but I'm pretty sure there's a bit of a difference IRL. That said, these characters should be really competent at scrounging and modifying stuff. Given the setting, a good number of us should be at least a little familiar with the craft of arrow-making. Enough that we could probably modify the arrows.
I wonder just how hard it would be to fire an arrow that's too big for your bow anyway. Anyone?