Whether or not spam is your intent, if you (or anyone) mass-add people to your list who haven't confirmed their subscription in compliance with our policy, there's always the possibility of accidental or malicious subscriptions. If you have proof that you have a list which already meets our requirements, we can mass-add it to our announcement list system for you (but you still can't use Mailman for announce-only lists).
Making it easy for people to unsubscribe is always a good idea - if someone gets unsubscribed, they can always re-subscribe. For the announcement lists, we do want some "hoops" for people to subscribe, but we definitely don't want them to have to jump through any hoops to unsubscribe.
If you have issues specific to your situation that require the use of Mailman for an announcement list and the list has a low number of subscribers, all of which have subscribed using some sort of confirmed opt-in system, write into support care of myself or Jeff; we're not inflexible, and it's possible that we can work something out for you.