Any type of mass migration isn't likely to happen unless Magento was somehow deemed off topic at Stack Overflow. Forget for a moment that Stack Overflow may not wish to have their content removed (and the hard-earned knowledge and reputation the users earned from it). You're not going to be able to build a comprehensive, functional community by migrating a bunch of questions en masse here. That would simply overwhelm a small up-start community, destroying any chances of establishing a scope and culture of your own.
Consider that, from day one, most of your migrated questions would become instantly orphaned as the original authors will be on another site. Your brand new site would be filled with questions largely linked as "anonymous." The original authors will not be notified or respond to requests for changes and improvements. Your tags will be inherited from another site, regardless of how this community wants to organize content here. Voting will not be commensurate with the size of the site. Comments would be largely meaningless and scattered with links to another site. Content would not be properly vetted and kept up until far off in the future when you are better able to curate that quantity of content.
A migration's primary purpose is to find a better home for questions that don't belong on the source site. I wouldn't rush to migrate a lot of older questions. Give yourselves some time to figure out the scope and culture of this site.
The idea of creating a new site is that you are proposing support for a community that is not being well-served by another site. Over time you may get new questions that have already been asked and covered well on Stack Overflow, but hopefully there's something about this community that will make your answers better than what you get over at Stack Overflow.
Otherwise, this site isn't making the Internet any better… and that's the point of being here in the first place.