The 'Export as CSV' feature lets you download all the posts - ones in the queue, ones that have been published, ones that have errors, and ones in the 'unscheduled posts' tab.

See how:

1. Filter the post by selecting any account or group.
2. Select the post you want to export, and click on the CSV button.
3. And your CSV will be downloaded.

You can schedule these posts later by just uploading this CSV.