First, during the early beta phase, it's better to avoid migrating questions to the site. The questions may turn out to be unwanted. They may come with answers that this community doesn't like, but migrated threads come with the votes from the original site. There should be no migration until this site has moderators, both because the moderators have the necessary means to counter bad migrations and as a benchmark for when the scope has probably settled down enough.
Furthermore this site needs to be seeded by content that reflects the community that is building it. It is a bad idea to shove in content from elsewhere. We're not trying to carve off a piece of Stack Overflow, we're trying to build a community around open source (or open knowledge or whatever).
In any case Questions can only be migrated during the first 60 days after posting. This doesn't make sense but it is clearly not going to change.
If a question has already been asked elsewhere, and it's relevant to this community, then feel free to ask it here, with attribution if necessary, and in a way that makes sense to this community. If there are interesting answers there, then feel free to post similar material here, with attribution if necessary, but again in a way that makes sense to this community.