You actually can, you just append @lemmy.world to the community name when accessing from another instance that’s federated with lemmy.world and once lemmy.world comes back up your contributions will be there. Any instance that’s federated with the instance your posting from will be able to participate in the discussion with you for that matter. The only thing you can’t do with a community when the host instance is down is subscribe to it. It would still get added to your subscriptions though if you try, the hosting insurance just won’t know until it comes back up and eats through the outboxes of federated instances to “catch up”.

Edit When it does come back up it’ll also get any messages that are in federated outboxes as well so your posts will ultimately show up on the host instance, just posted by your alt account