I really don't see how you would lengthen the limbs unless you built new ones. You could lengthen the riser to make the overall length of the bow longer, but I'm not sure what your trying to achieve? More forgiveness, more velocity, or more draw length?
I'm not a glass bow maker so I don't really have the answer, but I know some guys on here surely do. Just didn't want to leave you hangin out there with no replies.