The link below suggests that people with reputations of 3k or more can at least begin this process for a given thread
This question is more relevant to what you want to do: How to migrate old questions to a new graduated site.
However, no such large scale migrations will happen while the site is in beta, private or public. The beta period is the time for the community on this site to define what this site is going to be like and what questions are going to be accepted and answered here.
Don't worry about what's already answered on Stack Overflow. Focus on asking new, relevant questions and posting good, comprehensive answers on this site instead.