I've seen several questions recently where one very reasonable reason for closing would have been that they are more generic security questions, not really Unix specific. Is it possible to add migration there as a close reason?
There was a single migration in the last three months.
Even accounting for the fact that some migrations may be missed because nobody thinks of flagging, this is very far from warranting adding a new target to the migration dialog.
In addition there's a risk that questions will be migrated even though they're on-topic here, or worse, even though they're off-topic on the proposed destination. Questions about, say, configuring Apache for HTTPS or public key authentication for SSH, or setting up a firewall, are off-topic on Security.SE (because they're about configuring some tool, not about achieving a security objective), but Sec.SE frequently gets such question and has to migrate them away. If there was a migration path, I'd expect some of our community who are unfamiliar with the scope of Security.SE to see trigger words like “key” and “SSL” and migrate questions away even though they're on-topic here and off-topic on Sec.SE.
I am a regular both here and on Sec.SE. My subjective perception is that we do get the occasional question that would be on-topic on Sec.SE, but those are rarely off-topic here; when I see a question that is on-topic on both sides but would be better served on Sec.SE, I leave a comment suggesting that if the asker wishes they may have their question migrated. In the very rare cases when the question is actually off-topic here, I flag to migrate. I'm sure that I'm not the only one who does this. I doubt that many questions fall through the cracks, considering that I read more than half the questions here and I'm very likely to read a question if it's related to security.
In summary, no, we should not have a migration path to Security.SE, because it would be misused more often than useful.