Hi there,
I honestly don't know the exact answer to your question but I do know that you cannot buy the pass after you arrive in Japan. You must buy it before you enter through a local travel agent or authorized agent. When you are in Japan, you then exchange the receipt for the pass itself.
As to how it works if you are going to need to extend your stay... I don't know. I would really suggest you talk to your travel agent to confirm whether this is possible. Maybe someone else knows more...
