I don't think there is any direct way of getting fbx out of poser. Perhaps you could export to another app with COLLADA (via poser pro or D|S) and then export to fbx from it ? The 30 day demo of 3ds MAX could work for this, though i admit i haven't tried to do this before so YMMV.

And secondlife is one of those virtual life/online game type things and what usually people mean when they ask about 'avatars.' pjz99 probably thought that you wanted it for that.