Typically you’ll use an import script on a server that you control or a development instance. You would need to put your site if read only mode, download the backup from your hosted instance, do the work, make a backup, upload it to your hosted instance, and ask that your backup be restored. If you site is live and a day of read only time isn’t an issue do this.
If read only time is Really Bad and you have only a few thousand posts, write your script to have delay between calls and also retry when denied.
If you have tens of thousands of posts, you’ll need to convince yourself and whoever else that a day of read only time is not Really Bad
See scripts/import_scripts in the source code for examples.