First, we'd only want to do that for posts that have really good answers, not for any random post that happens to be about computer science.
Second, the question would have to be off-topic on Stack Overflow, otherwise it wouldn't be migrated.
Third, we still need to grow the site organically. A mass migration would not be good for us. We need to curate each post as it comes in, too — most incoming migrations need at a minimum to be retagged and have math formatting applied.
There is a lot of disagreement among the Stack Overflow community as to how much computer science is on-topic. Part of the community wants CS.SE not to exist as they don't recognize the existence of computer science as a science: they divide the world into programming (SO), and discussions (nowhere on SE). In practice, the SO community tends to want to keep good questions regardless of whether they're on-topic, and to want to get rid of bad questions regardless of whether they're on-topic.
In any case, it is now impossible to migrate questions that are more than 60 days old, even for moderators.
When you see a question on Stack Overflow that is off-topic there, flag or vote to close. If the question is less than 60 days old, and it's on-topic here, and it's a good question, then flag for migration — but be prepared for your flag to be either ignored (because if the question is closed in the meantime, that automatically marks your flag as helpful without a moderator ever seeing it) or declined (it depends which moderator handles your flag, they have different opinions on the scope of SO).
Don't go on a witch hunt on SO. Only flag posts that you stumble on organically.
If you see a good computer science question on SO that cannot be migrated (either because it straddles the border between programming and CS so that it's on-topic on SO as well, or because it's too old), feel free to ask it here in your own words. If you do that, you'll “take ownership” of the question in some sense: it'll be up to you to respond to comments, to mark an answer as accepted. Do link to the SO thread so that people can easily find prior work on the topic.